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GEM-STONES 



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I. SAI-I'HIRE 12. YELLOW SAI'FHIli 

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GEM-STONES 



GEM-STONES 

AND THEIR DISTINCTIVE CHARACTERS 



G. F. HERBERT SMITH 

M.A., D.Sc. 

OF THE BRITISH MUSEUM (NATURAL HISTORY) 



WITH MANY DIAGRAMS AND THIRTY-TWO PLATES 
OF WHICH THREE ARE IN COLOUR 



THIRD EDITION 



METHUEN & CO. LTD. 

36 ESSEX STREET W.G. 

LONDON 



First Published . . . March lit 
Second Edition . . . June 
Third Editum . 



PREFACE 

IN this edition the opportunity has been taken to 
correct a few misprints and mistakes that have 
been discovered in the first, and to alter slightly one 
or two paragraphs, but otherwise no change has been 
made. G. F. H. S. 

WANDSWORTH COMMON, S.W. 



PREFACE TO THE FIRST 
EDITION 

IT has been my endeavour to provide in this book 
a concise, yet sufficiently complete, account of 
the physical characters of the mineral species which 
find service in jewellery, and of the methods available 
for determining their principal physical constants to 
enable a reader, even if previously unacquainted with 
the subject, to have at hand all the information 
requisite for the sure identification of any cut stone 
which may be met with. For several reasons I have 
dealt somewhat more fully with the branches of 
science closely connected with the properties of 
crystallized matter than has been customary hitherto 
in even the most comprehensive books on precious 



2005117 



vi GEM-STONES 

stones. Recent years have witnessed many changes 
in the jewellery world. Gem-stones are no longer 
entirely drawn from a few well-marked mineral 
species, which are, on the whole, easily distinguishable 
from one another, and it becomes increasingly diffi- 
cult for even the most experienced eye to recognize 
a cut stone with unerring certainty. So long as the 
only confusion lay between precious stones and paste 
imitations an ordinary file was the solitary piece of 
apparatus required by the jeweller, but now recourse 
must be had to more discriminative tests, such as 
the refractive index or the specific gravity, the de- 
termination of which calls for a little knowledge and 
skill. Concurrently, a keener interest is being taken 
in the scientific aspect of gem-stones by the public 
at large, who are attracted to them mainly by 
aesthetic considerations. 

While the treatment has been kept as simple as 
possible, technical expressions, where necessary, have 
not been avoided, but their meanings have been 
explained, and it is hoped that their use will not 
prove stumbling-blocks to the novice. Unfamiliar 
words of this kind often give a forbidding air to a 
new subject, but they are used merely to avoid cir- 
cumlocution, and not, like the incantations of a 
wizard, to veil the difficulties in still deeper gloom. 
For actual practical work the pages on the refracto- 
meter and its use and the method of heavy liquids 
for the determination of specific gravities, and the 
tables of physical constants at the end of the book, 
with occasional reference, in case of doubt, to the 
descriptions of the several species alone are required ; 
other methods such as the prismatic mode of 
measuring refractive indices, or the hydrostatic way 



PREFACE vii 

of finding specific gravities which find a place in 
the ordinary curriculum of a physics course are 
described in their special application to gem-stones, 
but they are not so suitable for workshop practice. 
Since the scope of the book is confined mainly to 
the stones as they appear on the market, little has 
been said about their geological occurrence ; the case 
of diamond, however, is of exceptional interest and 
has been more fully treated. The weights stated 
for the historical diamonds are those usually pub- 
lished, and are probably in many instances far from 
correct, but they serve to give an idea of the sizes of 
the stones ; the English carat is the unit used, and 
the numbers must be increased by about 2\ per cent, 
if the weights be expressed in metric carats. The 
prices quoted for the various species must only be 
regarded as approximate, since they may change 
from year to year, or even day to day, according to 
the state of trade and the whim of fashion. 

The diagram on Plate II and most of the crystal 
drawings were made by me. The remaining draw- 
ings are the work of Mr. H. H. Penton. He likewise 
prepared the coloured drawings of cut stones which 
appear on the three coloured plates, his models, with 
two exceptions, being selected from the cut specimens 
.in the Mineral Collection of the British Museum by 
permission of the Trustees. Unfortunately, the 
difficulties that still beset the reproduction of pictures 
in colour have prevented full justice being done 
to the faithfulness of his brush. I highly appreciate 
the interest he took in the work, and the care and 
skill with which it was executed. My thanks are 
due to the De Beers Consolidated Mines Co. Ltd., 
and to Sir Henry A. Miers, F.R.S., Principal of the 



viii GEM-STONES 

University of London, for the illustrations of the 
Kimberley and Wesselton diamond mines, and of the 
methods and apparatus employed in breaking up 
and concentrating the blue ground ; to Messrs. I. J. 
Asscher & Co. for the use of the photograph of the 
Cullinan diamond ; to Mr. J. H. Steward for the loan 
of the block of the refractometer ; and to Mr. H. W. 
Atkinson for the illustration of the diamond-sorting 
machine. My colleague, Mr. W. Campbell Smith, 
B.A., has most kindly read the proof-sheets, and has 
been of great assistance in many ways. I hope that, 
thanks to his invaluable help, the errors in the book 
which may have escaped notice will prove few in 
number and unimportant in character. To Mr. 
Edward Hopkins I owe an especial debt of gratitude 
for his cheerful readiness to assist me in any way 
in his power. He read both the manuscript and the 
proof-sheets, and the information with regard to the 
commercial and practical side of the subject was 
very largely supplied by him. He also placed at 
my service a large number of photographs, some of 
which for instance, those illustrating the cutting of 
stones he had specially taken for me, and he pro- 
cured for me the jewellery designs shown on Plates 
IV and V. 

If this book be found by those engaged in the 
jewellery trade helpful in their everyday work, and 
if it wakens in readers generally an appreciation of 
the variety of beautiful minerals suitable for gems, 
and an interest in the wondrous qualities of crystal- 
lized substances, I shall be more than satisfied. 

G. F. H. S. 
WANDSWORTH COMMON, S.W. 



CONTENTS 



CHAP. PACK 

I. INTRODUCTION ..... i 



PART I SECTION A 
THE CHARACTERS OF GEM-STONES 

II. CRYSTALLINE FORM . . . . .6 

III. REFLECTION, REFRACTION, AND DISPERSION . 14 

IV. MEASUREMENT OF REFRACTIVE INDICES. . 21 
V. LUSTRE AND SHEEN . . . .37 

VI. DOUBLE REFRACTION . . . .40 
VII. ABSORPTION EFFECTS : COLOUR, DICHROISM, 

ETC 53 

VIII. SPECIFIC GRAVITY . . . .63 

IX. HARDNESS AND CLEAVABILITY . . .78 

X. ELECTRICAL CHARACTERS . . . .82 

PART I SECTION B 
THE TECHNOLOGY OF GEM-STONES 

XI. UNIT OF WEIGHT . . . . .84 

XII. FASHIONING OF GEM-STONES . . .88 

XIII. NOMENCLATURE OF PRECIOUS STONES . . 109 

XIV. MANUFACTURED STONES . . . .113 
XV. IMITATION STONES . . . . .124 

ix 



GEM-STONES 

PART II SECTION A 

PRECIOUS STONES 



CHAP. 

XVI. DIAMOND. 



XVII. OCCURRENCE OF DIAMOND . . 137 

XVIII. HISTORICAL DIAMONDS. . , 157 

XIX. CORUNDUM (Sapphire, Ruby) . . .172 

XX. BERYL (Emerald, Aquamarine, Morganite) . 184 



PART II SECTION B 
SEMI-PRECIOUS STONES 

XXI. TOPAZ 197 

XXII. SPINEL (Balas-Ruby, Rubicelle) . . 203 

XXIII. GARNET 207 

(a) HESSONITE (Grossular, Cinnamon-Stone, 

Hyacinth, Jacinth} . . .211 

(b) PYROPE (' Cape- Ruby'} . . .212 
(<r) RHODOLITE . . . .214 

(d) ALMANDINE (Carbuncle) . . .214 

(e) SPESSARTITE . . . .216 
(/) ANDRADITE (Demantoid, Topazolite, 

'Olivine'). . . . .216 

(g) UVAROVITE . . . .218 

XXIV. TOURMALINE (Rubellite) . . .219 

XXV. PERIDOT . . . . . .225 

XXVI. ZIRCON (Jargoon, Hyacinth, Jacinth] . . 228 

XXVII. CHRYSOBERYL (Chrysolite, Cats-Eye, Cymo- 

phane, Alexandrite) .... 233 

XXVIII. QUARTZ (Rock- Crystal, Amethyst, Citrine, 

Cairngorm, Cafs-Eye, Tigers-Eye) . . 238 

XXIX. CHALCEDONY, AGATE, ETC. . . . 246 



CONTENTS xi 

XXX. OPAL (White Opal, Black Opal, Fire-Opal) 249 
XXXI. FELSPAR (Moonstone, Sunstone, Labra- 

dorite, Amazon-Stone) . . -254 

XXXII. TURQUOISE, ODONTOLITE, VARISCITE . 257 

XXXIII. JADE (Nephrite or Greenstone, Jadeite) . 260 

XXXIV. SPODUMENE (Kunzite, Hiddenite], IOLITE, 

BENITOITE . . . . .265 

XXXV. EUCLASE, PHENAKITE, BERYLLONITE . 269 
XXXVI. ENSTATITE ('Green Garnet'}, DIOPSIDE, 
KYANITE, ANDALUSITE, IDOCRASE, EPI- 
DOTE, SPHENE, AXINITE, PREHNITE, 
APATITE, DIOPTASE . . .271 

XXXVII. CASSITERITE, ANATASE, PYRITES, HEMATITE 281 
XXXVIII. OBSIDIAN, MOLDAVITE . . .283 

PART II SECTION C 
ORNAMENTAL STONES 

XXXIX. FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, 
RHODONITE, AZURITE, MALACHITE, 
THULITE, MARBLE, APOPHYLLITE, 
CHRYSOCOLLA, STEATITE OR SOAPSTONE, 
MEERSCHAUM, SERPENTINE . . 285 



PART II SECTION D 
ORGANIC PRODUCTS 
XL. PEARL, CORAL, AMBER . . .291 

TABLES 

I. CHEMICAL COMPOSITION OF GEM-STONES . 300 
II. COLOUR OF GEM-STONES . . .301 

III. REFRACTIVE INDICES OF GEM-STONES . 302 



GEM-STONES 



PAG! 



IV. COLOUR-DISPERSION OF GEM-STONES . . 303 
V. CHARACTER OF THE REFRACTION OF GEM- 
STONES . . . . . -303 
VI. DICHROISM OF GEM-STONES . . . 304 
VII. SPECIFIC GRAVITIES OF GEM-STONES . . 305 
VIII. DEGREES OF HARDNESS OF GEM-STONES . 305 
IX. DATA . . . . . . .306 

INDEX ...... 307 



LIST OF PLATES 

PAGE 

I. GEM-STONES (in colour) . . . Frontispiece 

II. REFRACTIVE INDEX DIAGRAM. . . 36 

III. INTERFERENCE FIGURES . . .48 

IV. JEWELLERY DESIGNS . . . . 62 
V. JEWELLERY DESIGNS . . . .88 

VI. APPLIANCES USED FOR POLISHING DIAMONDS 102 

VII. POLISHING DIAMONDS . . . .103 

VIII. SLITTING AND POLISHING COLOURED STONES 104 

IX. FACETING MACHINE . . . .105 

X. LAPIDARY'S WORKSHOP AND OFFICE IN 

ENGLAND . . . . 106 

XL LAPIDARY'S WORKSHOP IN RUSSIA . . 107 

XII. FRENCH FAMILY CUTTING STONES . . 108 

XIII. INDIAN LAPIDARY . . . .109 

XIV. BLOWPIPE USED FOR THE MANUFACTURE 

OF RUBIES AND SAPPHIRES . . .118 

XV. KlMBERLEY MlNE, 1 87 1 . . . .140 

XVI. KlMBERLEY MINE, 1872. . . .141 

XVII. KlMBERLEY MlNE, 1874. . . .142 

XVIII. KlMBERLEY MlNE, l88l . . .143 

XIX. KlMBERLEY MlNE AT THE PRESENT DAY . 144 

XX. WESSELTON (open) MINE . . .145 

XXI. LOADING THE BLUE GROUND ON THE 

FLOORS, AND PLOUGHING IT OVER . . 146 
XXII. WASHING-MACHINES FOR CONCENTRATING 

THE BLUE GROUND . . . .147 

XXIII. DIAMOND-SORTING MACHINES. . . 148 
xiii 



xiv GEM-STONES 

PAGE 

XXIV. KAFFIRS PICKING OUT DIAMONDS . . 149 

XXV. CULLINAN DIAMOND (natural size) . . 168 

XXVI. LARGE AQUAMARINE CRYSTAL (one-sixth 
natural size), FOUND AT MARAMBAYA, 
MINAS GERAES, BRAZIL . . . .196 

XXVII. GEM-STONES (in colour) . . .226 

XXVIII. OPAL MINES, WHITE CLIFFS, NEW SOUTH 

WALES ...... 252 

XXIX. GEM-STONES (in colour) . . .256 

XXX. NATIVES DRILLING PEARLS . . . 294 

XXXI. METAL FIGURES OF BUDDHA INSERTED IN A 

PEARL-OYSTER . . . . 296 

XXXII. SECTIONS OF CULTURE PEARL . . 297 



GEM-STONES 



GEM-STONES 

CHAPTER 1 
INTRODUCTION 

BEAUTY, durability, and rarity: such are the 
three cardinal virtues of a perfect gem-stone. 
Stones lacking any of them cannot aspire to a high 
place in the ranks of precious stones, although it 
does not necessarily follow that they are of no use 
for ornamental purposes. The case of pearl, which, 
though not properly included among gem-stones, 
being directly produced by living agency, yet holds 
an honoured place in jewellery, constitutes to some 
extent an exception, since its incontestable beauty 
atones for its comparative want of durability. 

That a gem-stone should be a delight to the eye 
is a truism that need not be laboured ; for such is 
its whole raison d'etre. The members of the Mineral 
Kingdom that find service in jewellery may be 
divided into three groups, according as they are 
transparent, translucent, or opaque. Of these the 
first, which is by far the largest and the most 
important, may itself be further sub-divided into 
two sections: stones which are devoid of colour, 
and stones which are tinted. Among the former, 
diamond reigns supreme, since it alone possesses 



2 GEM-STONES 

that marvellous ' fire,' oscillating with every move- 
ment from heavenly blue to glowing red, which is 
so highly esteemed and so much besought. Other 
stones, such as ' fired ' zircon, white sapphire, white 
topaz, and rock-crystal, may dazzle with brilliancy 
of light reflected from the surface or emitted from 
the interior, but none of them, like diamond, glow 
with mysterious gleams. No hint of colour, save 
perhaps a trace of the blue of steel, can be tolerated 
in stones of this category ; above all is a touch of the 
jaundice hue of yellow abhorred. It taxes all the skill 
of the lapidary to assure that the disposition of the 
facets be such as to reveal the full splendour of the 
stone. A coloured stone, on the other hand, depends 
for its attractiveness more upon its intrinsic hue 
than upon the manner of its cutting. The tint must 
not be too light or too dark in shade : a stone that 
has barely any colour has little interest, and one 
which is too dark appears almost opaque and 
black. The lapidary can to some extent remedy 
these defects by cutting the former deep and the 
latter shallow. In certain curious stones for 
instance tourmaline the transparency, and in others 
such as ruby, sapphire, and one of the recent 
additions to the gem world, kunzite the colour, 
varies considerably in different directions. The 
colours that are most admired the fiery red of 
ruby, the royal blue of sapphire, the verdant green 
of emerald, and the golden yellow of topaz are 
pure tints, and the absorption spectra corresponding 
to them are on the whole continuous and often 
restricted. They therefore retain the purity of 
their colour even in artificial light, though certain 
sapphires transmit a relatively larger amount of red, 



INTRODUCTION 3 

and consequently turn purple at night. Of the 
small group of translucent stones which pass light, 
but are not clear enough to be seen through, the 
most important is opal. It and certain others 
of the group owe their merit to the same optical 
effect as that characterizing soap-bubbles, tarnished 
steel, and so forth, and not to any intrinsic coloration. 
Another set of stones moonstone and the star- 
stones reflect light from the interior more or less 
regularly, but not in such a way as to produce a 
play of colour. The last group, which comprises 
opaque stones, has a single representative among 
ordinary gem-stones, namely, turquoise. In this 
case light is scattered and reflected from layers 
immediately contiguous to the surface, and the 
colour is due to the resulting absorption. The 
apparent darkness of a deep-coloured stone follows 
from a different cause : the light passing into the 
stone is wholly absorbed within it, and, since none 
is emitted, the stone appears black. The claims of 
turquoise are maintained by the blue variety ; there 
is little demand for stones of a greenish tinge. 

It is evidently desirable that any stones used in 
jewellery should be able to resist the mechanical 
and chemical actions of everyday life. No one is 
anxious to replace jewels every few years, and the 
most valuable stones are expected to endure for 
all time. The mechanical abrasion is caused by 
the minute grains of sand that are contained in 
ordinary dust, and gem-stones should be at least 
as hard as they a condition fulfilled by all the 
principal species with the exception of opal, turquoise, 
peridot, and demantoid. Since the beauty of the 
first named does not depend on the brilliancy of its 



4 GEM-STONES 

polish, scratches on the surface are not of much 
importance ; further, all four are only slightly softer 
than sand. It may be noted that the softness of paste 
stones, apart from any objections that may be felt 
to the use of imitations, renders them unsuitable for 
jewellery purposes. The only stones that are likely 
to be chemically affected in the course of wear are 
those which are in the slightest degree porous. It 
is hazardous to immerse turquoises in liquids, even in 
water, lest the bluish green colour be oxidized to the 
despised yellowish hue. The risk of damage to opals, 
moonstones, and star-stones by the penetration of 
dirt or grease into the interior of the stones is less, 
but is not wholly negligible. Similar remarks apply 
with even greater force to pearls. Their charm, which 
is due to a peculiar surface-play of light, might be 
destroyed by contamination with grease, ink, or similar 
matter ; they are, moreover, soft. For both reasons 
their use in rings is much to be deprecated. Nothing 
can be more unsightly than the dingy appearance of 
a pearl ring after a few years' wear. 

It cannot be gainsaid that mankind prefers the 
rare to the beautiful, and what is within reach of 
all is lightly esteemed. It is for this reason that 
garnet and moonstone lie under a cloud. Purchasers 
can readily be found for a ' Cape-ruby ' or an 
'olivine,' but not for a garnet; garnets are so 
common, is the usual remark. Nevertheless, the 
stones mentioned are really garnets. If science 
succeeded in manufacturing diamonds at the cost 
of shillings instead of the pounds that are now asked 
for Nature's products not that such a prospect is 
at all probable or even feasible we might expect 
them to vanish entirely from fashionable jewellery. 



INTRODUCTION 5 

A careful study of the showcases of the most 
extensive jewellery establishment brings to light the 
fact that, despite the apparent profusion, the number 
of different species represented is restricted. 
Diamond, ruby, emerald, sapphire, pearl, opal, 
turquoise, topaz, amethyst are all that are ordinarily 
asked for. Yet, as later pages will show, there are 
many others worthy of consideration ; two among 
them peridot and tourmaline are, indeed, slowly 
becoming known. For the first five of the stones 
mentioned above, the demand is relatively steady, 
and varies absolutely only with the purchasing 
power of the world ; but a lesser known stone may 
suddenly spring into prominence owing to the caprice 
of fashion or the preference of some great lady or 
leader of fashion. Not many years ago, for instance, 
violet was the favourite colour for ladies' dresses, 
and consequently amethysts were much worn to 
match, but with the change of fashion they speedily 
sank to their former obscurity. Another stone may 
perhaps figure at some royal wedding; for a brief 
while it becomes the vogue, and afterwards is 
seldom seen. 

Except that diamond, ruby, emerald, and 
sapphire, and, we should add, pearl, may indis- 
putably be considered to occupy the first rank, it 
is impossible to form the gem-stones in any strict 
order. Every generation sees some change. The 
value of a stone is after all merely what it will 
fetch in the open market, and its artistic merits may 
be a matter of opinion. The familiar aphorism, 
de gustibus non est disputandum, is a warning not 
to enlarge upon this point. 



PART I SECTION A 

THE CHARACTERS OF GEM- 
STONES 

CHAPTER II 
CRYSTALLINE FORM 

WITH the single exception of opal, the whole 
of the principal mineral species used in 
jewellery are distinguished from glass and similar 
substances by one fundamental difference : they are 
crystallized matter, and the atoms composing them 
are regularly arranged throughout the structure. 

The words crystal and glass are employed in 
science in senses differing considerably from those 
in popular use. The former of them is derived 
from the Greek word pvo?, meaning ice, and was 
at one time used in that sense. For instance, the 
old fourteenth-century reading of Psalm cxlvii. 1 7, 
which appears in the authorized version as " He 
giveth his ice like morsels," ran " He sendis his 
kristall as morcels." It was also applied to the 
beautiful, lustrous quartz found among the eternal 
snows of the Alps, since, on account of their 
limpidity, these stones were supposed, as Pliny tells 
us, to consist of water congealed by the extreme 



CRYSTALLINE FORM 7 

cold of those regions ; such at the present day is 
the ordinary meaning of the word. But, when 
early investigators discovered that a salt solution 
on evaporation left behind groups of slender 
glistening prisms, each very similar to the rest, they 
naturally though, as we now know, wrongly 
regarded them as representing yet another form 
of congealed water, and applied the same word to 
such substances. Subsequent research has shown 
that these salts, as well as mineral substances 
occurring with natural faces in nature, have in 
common the fundamental property of regularity of 
arrangement of the constituent atoms, and science 
therefore defines by the word crystal a substance 
in which the structure is uniform throughout, and 
all the similar atoms composing it are arranged with 
regard to the structure in a similar way. 

The other word is yet more familiar ; it denotes 
the transparent, lustrous, hard, and brittle substance 
produced by the fusion of sand with soda or potash 
or both which fills our windows and serves a variety 
of useful purposes. Research has shown that 
glass, though apparently so uniform in character, 
has in reality no regularity of molecular arrange- 
ment. It is, in fact, a kind of mosaic of atoms, 
huddled together anyhow, but so irregular is its 
irregularity that it simulates perfect regularity. 
Science uses the word glass in this widened mean- 
ing. Two substances may, as a matter of fact, 
have the same chemical composition, and one be 
a crystal and the other a glass. For example, 
quartz, if heated to a high temperature, may be 
fused and converted into a glass. The difference 
in the two types of structure may be illustrated 



8 GEM-STONES 

by a comparison between a regiment of soldiers 
drawn up on parade and an ordinary crowd of 
people. 

The crystalline form is a visible sign of the 
molecular arrangement, and is intimately associated 
with the directional physical properties, such as the 
optical characters, cleavage, etc. A study of it is 
not only of interest in itself, but also of great 
importance to the lapidary who wishes to cut a stone 
to the best advantage, and it is, moreover, of service 
in distinguishing stones when in the rough state. 

The development of natural faces on a crystal 




FIG. I. Cubo-Octahedra. 

is far from being haphazard, but is governed by 
the condition that the angles between similar faces, 
whether on the same crystal or on different crystals, 
are equal, however varying may be the shapes and 
the relative sizes of the faces (Fig. i), and by the 
tendency of the faces bounding the crystal to fall 
into series with parallel edges, such series being 
termed zones. Each species has a characteristic 
type of crystallization, which may be referred to 
one of the following six systems : 

I. Cubic. Crystals in this system can be re- 
ferred to three edges, which are mutually at right 
angles, and in which the directional characters are 
identical in value. These principal edges are known 



CRYSTALLINE FORM 



as axes. Some typical forms are the cube (Fig. 2), 
characteristic of fluor ; the octahedron (Fig. 3), 
characteristic of diamond and spinel ; the dodeca- 
hedron (Fig. 4), characteristic of garnet; and the 




FIG. 



Cube. FIG. 3. Octahedron. FIG. 4. Dodecahedron. 



triakisoctahedron, or three-faced octahedron (Fig. 5). 

All crystals belonging to this system are singly 
refractive. 

2. Tetragonal. Such crystals can be referred 




FIG. 5. Triakis- 
octahedron, or 
Three-faced Oc- 
tahedron. 



FIG. b. Tetra- 
gonal Crystal. 



to three axes, which are mutually at right angles, 
but in only two of them are the directional characters 
identical. A typical form is a four-sided prism, 
mm, of square section, terminated by four triangular 
faces,/* (Fig. 6), the usual shape of crystals of zircon 
and idocrase. 



10 



GEM-STONES 



Crystals belonging to this system are doubly 
refractive and uniaxial, i.e. they have one direction 
of single refraction (cf. p. 45), which is parallel to 
the unequal edge of the three mentioned above. 

H 3. Hexagonal.- Such crystals 

can be referred alternatively 
either to a set of three axes, 
X, Y, Z (Fig. 7), which lie in 
a plane perpendicular to a fourth, 
H, and are mutually inclined at 
angles of 60, or to a set of 
three, a, b, c, which are not at 
FIG. 7.-T wo alternative r igh t angles as in the cubic 
system, but in which the direc- 
tional characters are identical. 



sets of Axes in the 
Hexagonal System. 



The fourth axis in the first arrangement is equally 
inclined to each in the second set of axes. Many 
important species crystallize in 
this system corundum (sapphire, 
ruby), beryl (emerald, aqua- 
marine), tourmaline, quartz, and 
phenakite. The crystals usually 




FIGS. 8-10. Hexagonal Crystals. 



display a six-sided prism, terminated by a single 
face, c, perpendicular to the edge of the prism m 
(Fig. 8), e.g. emerald, or by six or twelve inclined 
faces, p (Fig. 9), e.g. quartz, crystals of which are 



CRYSTALLINE FORM 



1 1 



so constant in form as to be the most familiar in the 
Mineral Kingdom. Tourmaline crystals (Fig. 10) 
are peculiar because of the fact that often one end 
is obviously to the eye flatter than the other. 

Crystals belonging to this system are also doubly 
refractive and uniaxial, the direction of single 
refraction being parallel to the fourth axis mentioned 
above, and therefore also parallel to the prism edge. 
Hence deeply coloured tourmaline, which strongly 
absorbs the ordinary ray, must be cut with the 
table-facet parallel to the edge 
of the prism. 

4. Orthorhombic. Such 
crystals can be referred to 
three axes, which are mutu- 
ally at right angles, but in 
which each of the directional 
characters are different. The 

crystals are usually prismatic FlG> Vi. -Relation of the 

in shape, One of the axes two directions of single 

being parallel to the prism Refraction to the Axes 

, _ 1 . j in an Orthorhombic 

edge. Topaz, peridot, and Crystal 

chrysoberyl are the most 

important species crystallizing in this system. 

Crystals belonging to this system are doubly 
refractive and biaxial, i.e. they have two directions 
of single refraction (cf. p. 45). The three axes 
a, d, c (Fig. 1 1) are parallel respectively to the two 
bisectrices of the directions of single refraction, 
and the direction perpendicular to the plane con- 
taining those directions. 

5. Monoclinic. Such crystals can be referred to 
three axes, one of which is at right angles to the 
other two, which are, however, themselves not at 




12 GEM-STONES 

right angles. Spodumene (kunzite) and some 
moonstone crystallize in this system. 

Crystals belonging to this system are doubly 
refractive and biaxial, but in this case the first axis 
alone is parallel to one of the principal optical 
directions. 

6. Tridinic. Such crystals have no edges at 
right angles, and the optical characters are not 
immediately related to the crystalline form. Some 
moonstone crystallizes in this system. 

Crystals are often not single separate individuals. 
For instance, diamond and spinel are found in fiat 
triangular crystals with their girdles 
cleft at the corners (Fig. 1 2). Each 
of such crystals is really composed 
of portions of two similar octahedra, 
which are placed together in such 
a way that each is a reflection of 
FIG. i2. Twinned the other. Such composite crystals 
Octahedron. are called twins or macles. Some- 
times the twinning is repeated, and 
the individuals may be so minute as to call for a 
microscope for their perception. 

A composite structure may also result from the 
conjunction of numberless minute individuals 
without any definite orientation, as in the case of 
chalcedony and agate. So by supposing the 
individuals to become infinitesimally small, we pass 
to a glass-like substance. 

It is often a peculiarity of crystals of a species 
to display a typical combination of natural faces. 
Such a combination is known as the habit of the 
species, and is often of service for the purpose of 
identifying stones before they are cut. Thus, a 




CRYSTALLINE FORM 13 

habit of diamond and spinel is an octahedron, often 
twinned, of garnet a dodecahedron, of emerald a 
flat-ended hexagonal prism, and so on. 

It is one of the most interesting and remarkable 
features connected with crystallization that the 
composition and the physical characters for instance, 
the refractive indices and specific gravity may, 
without any serious disturbance of the molecular 
arrangement, vary considerably owing to the more 
or less complete replacement of one element by 
another closely allied to it. That is the cause of 
the range of the physical characters which has been 
observed in such species as tourmaline, peridot, 
spinel, etc. The principal replacements in the 
case of the gem-stones are ferric oxide, Fe 2 O 3 , by 
alumina, A1 2 O 3 , and ferrous oxide, FeO, by magnesia, 
MgO. 

A list of the principal gem-stones, arranged by 
their chemical composition, is given in Table I at 
the end of the book. 



CHAPTER III 
REFLECTION, REFRACTION, AND DISPERSION 

IT is obvious that, since a stone suitable for 
ornamental use must appeal to the eye, its 
most important characters are those which depend 
upon light ; indeed, the whole art of the lapidary 
consists in shaping it in such a way as to show 
these qualities to the best advantage. To under- 
stand why certain forms are given to a cut stone, 
it is essential for us to ascertain what becomes of 
the light which falls upon the surface of the stone ; 
further, we shall find that the action of a stone upon 
light is of very great help in distinguishing the 
different species of gem-stones. The phenomena 
displayed by light which impinges upon the surface 
separating two media 1 are very similar in character, 
whatever be the nature of the media. 

Ordinary experience with a plane mirror tells us 
that, when light is returned, or reflected, as it is 
usually termed, from a plane or flat surface, there 
is no alteration in the size of objects viewed in this 
way, but that the right and the left hands are inter- 
changed : our right hand becomes the left hand in 

1 The word medium is employed by physicists to express any sub- 
stance through which light passes, and includes solids such as glass, 
liquids such as water, and gases such as air ; the nature of the substance 
is not postulated. 



REFLECTION, REFRACTION, DISPERSION 15 

our reflection in the mirror. We notice, further, that 
our reflection is apparently just as far distant from 
the mirror on the farther side as we are on this 
side. In Fig. 1 3 MM' is a section of the mirror, 
and O' is the image of the hand O as seen in the 
mirror. Light from O reaches the eye E by way 
of m, but it appears to come from O. Since OO 
is perpendicular to the mirror, and O and O lie at 
equal distances from it, it follows from elementary 




FIG. 13. Reflection at a Plane Mirror. 

geometry that the angle z", which the reflected ray 
makes with win, the normal to the mirror, is equal 
to 2, the angle which the incident ray makes with 
the same direction. 

Again, everyday experience tells us that the 
case is less simple when light actually crosses the 
bounding surface and passes into the other medium. 
Thus, if we look down into a bath filled with water, 
the bottom of the bath appears to have been raised 
up, and a stick plunged into the water seems to be 



1 6 GEM-STONES 

bent just at the surface, except in the particular 
case when it is perfectly upright. Since the stick 
itself has not been bent, light evidently suffers some 
change in direction as it passes into the water or 
emerges therefrom. The passage of light from one 
medium to another was studied by Snell in the 
seventeenth century, and he enunciated the follow- 
ing laws : 

1. The refracted ray lies in the plane containing 
the incident ray and the normal to the plane surface 
separating the two media. 

It will be noticed that the reflected ray obeys 
this law also. 

2. The angle r, which the refracted ray makes 
with the normal, is related to the angle z, which the 
incident ray makes with the same direction, by the 
equation 

n sin i = n sin r, (a) 

where n and n are constants for the two media 
which are known as the indices of refraction, or the 
refractive indices. 

This simple trigonometrical relation may be ex- 
pressed in geometrical language. Suppose we cut 
a plane section through the two media at right 
angles to the bounding plane, which then appears 
as a straight line, SOS' (Fig. 14), and suppose that 
IO represents the direction of the incident ray ; then 
Snell's first law tells us that the refracted ray OR 
will also lie in this plane. Draw the normal NON 1 , 
and with centre O and any radius describe a circle 
intersecting the incident and refracted rays in the 
points a and b respectively ; let drop perpendiculars 
ac and bd on to the normal NON'. Then we have 



REFLECTION, REFRACTION, DISPERSION if 

n .acri . bd, whence we see that if n be greater than 
', ac is less than bd, and therefore when light passes 
from one medium into another which is less optically 
dense, in its passage across the boundary it is bent, 
or refracted, away from the normal. 

We see, then, that when light falls on the boundary 
of two different media, some is reflected in the first 
and some is refracted into the second medium. 







N 

FIG. 14. Refraction across a Plane Surface. 

The relative amounts of light reflected and refracted 
depend on the angle of incidence and the refractive 
indices of the media. We shall return to this point 
when we come to consider the lustre of stones. 

We will proceed to consider the course of rays at 
different angles of incidence when light passes out 
from a medium into one less dense for instance, 
from water into air. Corresponding to light with 
a small angle of incidence such as I^O (Fig. 15), 
some of it is reflected in the direction OI\ and the 

2 



i8 



GEM-STONES 



remainder is refracted out in the direction OR V 
Similarly, for the ray 7 2 <9 some is reflected along 
0/2 and some refracted along OR Z . Since, in the 
case we have taken, the angle of refraction is 
greater than the angle of incidence, the refracted 
ray corresponding to some incident, ray I C O will 
graze the bounding surface, and corresponding to 




Ic 



FIG. 15. Total -Reflection. 

a ray beyond it, such as 7 3 (9, which has a still greater 
angle of incidence, there is no refracted ray, and 
all the light is wholly or totally reflected within the 
dense medium. The critical angle I C ON, which is 
called the angle of total-reflection, is very simply 
related to the refractive indices of the two media ; 
for, since r is now a right angle, sin r= i, and equa- 
tion (a) becomes 

n sin i n' . (b\ 



REFLECTION, REFRACTION, DISPERSION 19 

Hence, if the angle of total-reflection is measured 
and one of the indices is known, the other can easily 
be calculated. 

The phenomenon of total-reflection may be ap- 
preciated if we hold a glass of water above our head, 
and view the light of a lamp on a table reflected 
from the under surface of the water. This reflection 
is incomparably more brilliant than that given by 
the upper surface. 

The refractive index of air is taken as unity ; 
strictly, it is that of a vacuum, but the difference 
is too small to be appreciated even in very delicate 
work. Every substance has different indices for 
light of different colour, and it is customary to take 
as the standard the yellow light of a sodium flame. 
This happens to be the colour to which our eyes are 
most sensitive, and a flame of this kind is easily 
prepared by volatilizing a little bicarbonate of soda 
in the flame of a bunsen burner. A survey of 
Table III at the end of the book shows clearly how 
valuable a measurement of the refractive index is 
for determining the species to which a cut stone 
belongs. The values found for different specimens 
of the species do in cases vary considerably owing 
to the great latitude possible in the chemical con- 
stitution due to the isomorphous replacement of one 
element by another. Some variation in the index 
may even occur in different directions within the 
same stone ; it results from the remarkable 
property of splitting up a beam of light into two 
beams, which is possessed by many crystallized 
substances. This forms the subject of a later 
chapter. 

Upon the fact that the refractive index of a 



20 GEM-STONES 

substance varies for light of different colours depends 
such familiar phenomena as the splendour of the 
rainbow and the ' fire ' of the diamond. When 
white light is refracted into a stone it no longer 
remains white, but is split up into a spectrum. 
Except in certain anomalous substances the refractive 
index increases progressively as the wave-length of 
the light decreases, and consequently a normal 
spectrum is violet at one end and passes through 
green and yellow to red at the other end. The width 
of the spectrum, which may be measured by the 
difference between the refractive indices for the 
extreme red and violet rays, also varies, though on 
the whole it increases with the refractive index. It 
is the dispersion, as this difference is termed, that 
determines the ' fire ' a character of the utmost 
importance in colourless transparent stones, which, 
but for it, would be lacking in interest. Diamond 
excels all colourless stones in this respect, although 
it is closely followed by zircon, the colour of which 
has been driven off by heating ; it is, however, sur- 
passed by two coloured species : sphene, which is 
seldom seen in jewellery, and demantoid, the green 
garnet from the Urals, which often passes under the 
misnomer ' olivine.' The dispersion of the more 
prominent species for the B and G lines of the solar 
spectrum is given in Table IV at the end of the 
book. 

We will now proceed to discuss methods that 
may be used for the measurement of the refractive 
indices of cut stones. 



CHAPTER IV 
MEASUREMENT OF REFRACTIVE INDICES 

THE methods available for the measurement 
of refractive indices are of two kinds, and 
make use of two different principles. The first, 
which is based upon the very simple relation found 
in the last chapter to subsist at total-reflection, 
can be used with ease and celerity, and is best 
suited for discriminative purposes ; but it is re- 
stricted in its application. The second, which 
depends on the measurement of the angle between 
two facets and the minimum deviation experienced 
by a ray of light when traversing a prism formed by 
them, is more involved, entails the use of more 
elaborate apparatus, and takes considerable time, 
but it is less restricted in its application. 

(i) THE METHOD OF TOTAL-REFLECTION 

We see from equation b (p. 1 8), connecting the 
angle of total-reflection with the refractive indices 
of the adjacent media, that, if the denser medium 
be constant, the indices of all less dense media 
may be easily determined from a measurement of 
the corresponding critical angle. In all refracto- 
meters the constant medium is a glass with a high 
refractive index. Some instruments have rotatory 



22 GEM-STONES 

parts, by means of which this angle is actually 
measured. Such instruments give very good 
results, but suffer from the disadvantages of being 
neither portable nor convenient to handle, and of 
not giving a result without some computation. 

For use in the identification of cut stones, a 
refractometer with a fixed scale, such as that (Fig. 
1 6) devised by the author, is far more convenient. 
In order to facilitate the observations, a totally 
reflecting prism has been inserted between the two 




FlG. 16. Refractometer (actual size). 

lenses of the eyepiece. The eyepiece may be 
adjusted to suit the individual eyesight; but for 
observers with exceptionally long sight an adapter 
is provided, which permits the eyepiece being 
drawn out to the requisite extent. The refracto- 
meter must be held in the manner illustrated in 
Fig. 17, so that the light from a window or other 
source of illumination enters the instrument by the 
lenticular opening underneath. Good, even illumina- 
tion of the field may also very simply be secured 
by reflecting light into the instrument from a sheet 



REFRACTIVE INDICES 23 

of white paper laid on a table. On looking down 
the eyepiece we see a scale (Fig. 1 8), the eyepiece 
being, if necessary, focused until the divisions of 
the scale are clearly and distinctly seen. Suppose, 
for experiment, we smear a little vaseline or similar 
fatty substance on the plane surface of the dense 
glass, which just projects beyond the level of the 




FIG. 17. Method of Using the Refractometer. 

brass plate embracing it. The field of view is now 
no longer uniformly illuminated, but is divided 
into two parts (Fig. 19): a dark portion above, 
which terminates in a curved edge, apparently 
green in colour, and a bright portion underneath, 
which is composed of totally reflected light. If 
we tilt the instrument downwards so that light 
enters the instrument from above through the 
vaseline we find that the portions of the field are 



GEM-STONES 



reversed, the dark portion being underneath and 
terminated by a red edge. It is possible so to 
arrange the illumination that the two portions 
are evenly lighted, and the common edge becomes 
almost invisible. It is therefore essential for 
obtaining satisfactory results that the plate and 
the dense glass be shielded from the light by the 



IEFFWCTIVE INDEX 

-= 1-30 

m 1-35 

fH 1-40 

fJH 1-45 

= 1-50 

HI 1-55 

= 1-60 



1-70 



= 1-75 




FIG. 1 8. Scale 
of the Refrac- 
tometer. 



FIG. 19. Shadow, 
edge given by a 
singly refractive 
Substance. 



disengaged hand. The shadow-edge is curved, 
and is, indeed, an arc of a circle, because spherical 
surfaces are used in the optical arrangements of 
the refractometer ; by the substitution of cylindrical 
surfaces it becomes straight, but sufficient advantage 
is not secured thereby to compensate for the greatly 
increased complexity of the construction. The 
shadow-edge is coloured, because the relative 

dispersion, (n v and n r being the refractive 



REFRACTIVE INDICES 25 

indices for the extreme violet and red rays 
respectively), of the vaseline differs from that of 
the dense glass. The dispersion of the glass is 
very high, and exceeds that of any stone for 
which it can be used. Certain oils have, however, 
nearly the same relative dispersion, and the edges 
corresponding to them are consequently almost 
colourless. A careful eye will perceive that the 
coloured shadow-edge is in reality a spectrum, of 
which the violet end lies in the dark portion of 
the field and the red edge merges into the bright 
portion. The yellow colour of a sodium flame, 
which, as has already been stated, is selected as 
the standard for the measurement of refractive 
indices, lies between the green and the red, and 
the part of the spectrum to be noted is at the 
bottom of the green, and practically, therefore, at 
the bottom of the shadow, because the yellow and 
red are almost lost in the brightness of the lower 
portion of the field. If a sodium flame be used 
as the source of illumination, the shadow-edge 
becomes a sharply defined line. The scale is so 
graduated and arranged that the reading where 
this line crosses the scale gives the corresponding 
refractive index, the reading, since the line is 
curved, being taken in the middle of the field on 
the right-hand side of the scale. The refracto- 
meter therefore gives at once, without any inter- 
mediate calculation, a value of the refractive index 
to the second place of decimals, and a skilled 
observer may, by estimating the tenths of the 
intervals between successive divisions, arrive at 
the third place ; to facilitate this estimation the 
semi-divisions beyond 1-650 have been inserted. 



26 GEM-STONES 

The range extends nearly to r8oo; for any 
substance with a higher refractive index the field 
is dark as far as the limit at the bottom. 

A fat, or a liquid, wets the glass, i.e, comes into 
intimate contact with it, but if a solid substance 
be tested in the same way, a film of air would 
intervene and entirely prevent an observation. To 
displace it, a drop of some liquid which is more 
highly refractive than the substance under test 
must first be applied to the plane surface of the 
dense glass. The most convenient liquid for the 
purpose is methylene iodide, CH 2 I 2 , which, when 
pure, has at ordinary room temperatures a refrac- 
tive index of 1*742. It is almost colourless when 
fresh, but turns reddish brown on exposure to light. 
If desired, it may be cleared in the manner described 
below (p. 66), but the film of liquid actually used 
is so thin that this precaution is scarcely necessary. 
If we test a piece of ordinary glass one of the slips 
used by microscopists for covering thin sections is 
very convenient for the purpose first applying a 
drop of methylene iodide to the plane surface of the 
dense glass of the refractometer (Fig. 20), we notice 
a coloured shadow-edge corresponding to the glass- 
slip at about i'53O and another, almost colourless, 
at 1742, which corresponds to the liquid. If the 
solid substance which is tested is more highly 
refractive than methylene iodide, only the latter 
of the shadow-edges is visible, and we must utilize 
some more refractive liquid. We can, however, 
raise the refractive index of methylene iodide by 
dissolving sulphur l in it ; the refractive index of 

1 Methylene iodide must be heated almost to boiling-point to enable 
it to absorb sufficient sulphur ; but caution must be exercised in the 



REFRACTIVE INDICES 27 

the saturated liquid lies well beyond i'8oo, and 
the shadow-edge corresponding to it, therefore, does 
not come within the range of the refractometer. 
The pure and the saturated liquids can be procured 
with the instrument, the bottles containing them 
being japanned on the outside to exclude light and 
fitted with dipping-stoppers, by means of which a 
drop of the liquid required is easily transferred to 
the surface of the glass of the instrument. So long 



Stone 




FIG. 20. Faceted Stone in Position on the Refractometer. 

as the liquid is more highly refractive than the stone, 
or whatever may be the substance under examination, 
its precise refractive index is of no consequence. The 
facet used in the test must be flat, and must be 
pressed firmly on the instrument, so that it is truly 
parallel to the plane surface of the dense glass ; for 
good results, moreover, it must be bright. 

operation to prevent the liquid boiling over and catching fire, the 
resulting fumes being far from pleasant. It is advisable to verify by 
actual observation that the liquid is refractive enough not to show any 
shadow-edge in the field of view of the refractometer. 



28 GEM-STONES 

We have so far assumed that the substance 
which we are testing is simple and gives a 
single shadow-edge; but, as may be seen from 
Table V, many of the gem-stones are doubly 
refractive, and such will, in general, show in the 
field of the refractometer two distinct shadow- 
edges more or less widely separated. Suppose, 
for example, we study the effect produced by a 
peridot, which displays the phenomenon to a 
marked degree. If we revolve the stone so that 
the facet under observation remains parallel to 
the plane surface of the dense glass of the refracto- 
meter and in contact with it, we notice that both 
the shadow-edges in general move up or down 
the scale. In particular cases, depending upon the 
relation of the position of the facet selected to 
the crystalline symmetry, one or both of them 
may remain fixed, or one may even move across 
the other. But whatever facet of the stone be 
used for the test, and however variable be the 
movements of the shadow -edges, the highest and 
lowest readings obtainable remain the same; they 
are the principal indices of refraction, such as are 
stated in Table III at the end of the book, and 
their difference measures the maximum amount of 
double refraction possessed by the stone. The 
procedure is therefore simplicity itself; we have 
merely to revolve the stone on the instrument, 
usually through not more than a right angle, and 
note the greatest and least readings. It will be 
noticed that the shadow-edges cross the scale 
symmetrically in the critical and skewwise in 
intermediate positions. Fig. 21 represents the 
effect when the facet is such as to give simul- 



REFRACTIVE INDICES 



29 




taneously the two readings required. The shadow- 
edges a and b, which are coloured in white light, 
correspond to the least and greatest respectively 
of the principal refractive indices, while the third 
shadow-edge, which is very faint, corresponds to 
the liquid used methylene iodide. It is possible, 
as we shall see in a later chapter, to learn from 
the motion, if any, of the shadow- 
edges something as to the character 
of the double refraction. Since, 
however, each shadow-edge is spec- 
tral in white light, they will not be 
distinctly separate unless the double 
refraction exceeds the relative dis- 
persion. Topaz, for instance, ap- 
pears in white light to yield only 
a single shadow-edge, and may thus 
easily be distinguished from tour- 
maline, in which the double re- 
fraction is large enough for the 
separation of the two shadow-edges 
to be clearly discerned. In sodium 
light, however, no difficulty is ex- 
perienced in distinguishing both the 
shadow-edges given by substances with small amount 
of double refraction, such as chrysoberyl, quartz, and 
topaz, and a skilled observer may detect the separa- 
tion in the extreme instances of apatite, idocrase, 
and beryl. The shadow-edge corresponding to the 
greater refractive index is always less distinct, 
because it lies in the bright portion of the field. 
If the stone or its facet be small, it must be moved 
on the plane surface of the dense glass until the 
greatest possible distinctness is imparted to the 



FIG. 21. Shadow- 
edges given by a 
doubly refractive 
Substance. 



30 GEM-STONES 

edge or edges. If it be moved towards the 
observer from the further end, a misty shadow 
appears to move down the scale until the correct 
position is reached, when the edges spring into 
view. 

Any facet of a stone may be utilized so long as 
it is flat, but the table-facet is the most convenient, 
because it is usually the largest, and it is available 
even when the stone is mounted. That the stone 
need not be removed from its setting is one of 
the great advantages of this method. The smaller 
the stone the more difficult it is to manipulate ; 
caution especially must be exercised that it be 
not tilted, not only because the shadow-edge would 
be shifted from its true position and an erroneous 
value of the refractive index obtained, but also 
because a corner or edge of the stone would 
inevitably scratch the glass of the instrument, 
which is far softer than the hard gem-stones. 
Methylene iodide will in time attack and stain the 
glass, and must therefore be wiped off the instru- 
ment immediately after use. 

(2) THE METHOD OF MINIMUM DEVIATION 

If the stone be too highly refractive for a 
measurement of its refractive index to be possible 
with the refractometer just described, and it is 
desired to determine this constant, recourse must 
be had to the prismatic method, for which purpose 
an instrument known as a goniometer l is required. 

1 yuvla, angle ; /j.4rpov, measure. For details of the construction, 
adjustment, and use of this instrument the reader should refer to text- 
books of mineralogy or crystallography. 



REFRACTIVE INDICES 31 

Two angles must be measured ; one the interior 
angle included between a suitable pair of facets, 
and the other the minimum amount of the deviation 
produced by the pair upon a beam of light 
traversing them. 

Fig. 22 represents a section of a step-cut stone 
perpendicular to a series of facets with parallel 
edges ; / is the table, and a, b, c, are facets on 
the culet side. The path of light traversing the 
prism formed by the pair 
of facets, / and b, is 
indicated. Suppose that 
A is the interior angle 
of the prism, i the angle 
of incidence of light at 
the first facet and if the 
angle of emergence at 
the second facet, and r 
and / the angles inside 
the stone at the two facets 
respectively. Then at the FIG. 22. Path at Minimum De- 
first facet light has been 
bent through an angle a Cut Stone"." 
i r, and again at the 

second facet through an angle i' - / ; the angle of 
deviation, D, is therefore given by 




We have further that 

whence it follows that 

A + D~i+?. 

If the stone be mounted on the goniometer 



32 GEM-STONES 

and adjusted so that the edge of the prism is 
parallel to the axis of rotation of the instrument 
and if light from the collimator fall upon the 
table-facet and the telescope be turned to the 
proper position to receive the emergent beam, a 
spectral image of the object-slit, or in the case of 
a doubly refractive stone in general, two spectral 
images, will be seen in white light ; in the light 
of a sodium flame the images will be sharp and 
distinct. Suppose that we rotate the stone in 
the direction of diminishing deviation and simul- 
taneously the telescope so as to retain an image in 
the field of view, we find that the image moves 
up to and then away from a certain position, at 
which, therefore, the deviation is a minimum. The 
image moves in the same direction from this 
position whichever way the stone be rotated. 
The question then arises what are the angles 
of incidence and refraction under these special 
conditions. It is clear that a path of light is 
reversible ; that is to say, if a beam of light 
traverses the prism from the facet t to the facet b, 
it can take precisely the same path from the facet 
b to the facet t. Hence we should be led to 
expect that, since experiment teaches us that there 
is only one position of minimum deviation corre- 
sponding to the same pair of facets, the angles at the 
two facets must be equal, i.e. i=i f , and rS. It 
is, indeed, not difficult to prove by either geometrical 

or analytical methods that such is the case. 

^0 

Therefore at minimum deviation r= and 

2 

. A+D . . 

t = , and, since sin t = n sm r, where is 



REFRACTIVE INDICES 33 

the refractive index of the stone, we have the 
simple relation 




This relation is strictly true only when the 
direction of minimum deviation is one of crystal- 
line symmetry in the stone, and holds therefore 
in general for all singly refractive stones, and for 
the ordinary ray of a uniaxial stone ; but the 
values thus obtained even in the case of biaxial 
stones are approximate enough for discriminative 
purposes. If then the stone be singly refractive, 
the result is the index required ; if it be uniaxial, 
one value is the ordinary index and the other 
image gives a value lying between the ordinary 
and the extraordinary indices ; if it be biaxial, the 
values given by the two images may lie anywhere 
between the greatest and the least refractive indices. 
The angle A must not be too large ; otherwise the 
light will not emerge at the second facet, but will 
be totally reflected inside the stone : on the other 
hand, it must not be too small, because any error 
in its determination would then seriously affect the 
accuracy of the value derived for the refractive 
index. Although the monochromatic light of a 
sodium flame is essential for precise work, a 
sufficiently approximate value for discriminative 
purposes is obtained by noting the position of the 
yellow portion of the spectral image given in white 
light. 

In the case of a stone such as that depicted in 
Fig. 2 2 images are given by other pairs of facets, for 
3 



34 



GEM-STONES 



instance ta and tc, unless the angle included by 
the former is too large. There might therefore be 
some doubt, to which pair some particular image 
corresponded; but no confusion can arise if the 
following procedure be adopted. 

The table, or some easily recognizable facet, 
is selected as the facet at which light enters the 
stone. The telescope is first placed in the position 
in which it is directly opposite the collimator 
(T in Fig. 23), and clamped. The scale is turned 

until it reads ex- 
actly zero, o or 
360, and clamped. 
The telescope is re- 
leased and revolved 
in the direction of 
T * increasing readings 
of the scale to the 
position of minimum 
deviation, T. The 

reading of the scale 
FIG. 23. Course of Observations in the . 

Method of Minimum Deviation. g lves at once the 

angle of minimum 

deviation, D. The holder carrying the stone is 
now clamped to the scale, and the telescope is 
turned to the position, 7\, in which the image 
given by reflection from the table facet is in the 
centre of the field of view; the reading of the scale 
is taken. The telescope is clamped, and the scale 
is released and rotated until it reads the angle 
already found for D. If no mistake has been made, 
the reflected image from the second facet is now 
in the field of view. It will probably not be quite 
central, as theoretically it should be, because the 




REFRACTIVE INDICES 35 

stone may not have been originally quite in the 
position of minimum deviation, a comparatively 
large rotation of the stone producing no apparent 
change in the position of the refracted image at 
minimum deviation, and further, because, as has 
already been stated, the method is not strictly true for 
biaxial stones. The difference in readings, however, 
should not exceed 2. The reading, S, of the 
scale is now taken, and it together with 180 
subtracted from the reading for the first facet, and 
the value of A, the interior angle between the two 
facets, obtained. 

Let us take an example. 

Reading T ( = /?) 40 41' Reading 7\ 261 35' 

less 1 80 i So o 



8i 35 
Reading 5" 41 30 



\D 20 2oJ A 40 

\A 20 2\ \A 20 



o 23 

Log sin 40 23' 9.81151 
Log sin 20 2| 9.53492 

Log n 0.27659 

n= 1.8906. 

The readings 5" and T are very nearly the same, 
and therefore we may be sure that no mistake 
has been made in the selection of the facets. 

In place of logarithm-tables we may make use 
of the diagram on Plate II. The radial lines 



36 GEM-STONES 

correspond to the angles of minimum deviation 
and the skew lines to the prism angles, and the 
distance along the radial lines gives the refractive 
index. We run our eye along the line for the 
observed angle of minimum deviation and note 
where it meets the curve for the observed prism 
angle ; the refractive index corresponding to the 
point of intersection is at once read off. 

This method has several obvious disadvantages : 
it requires the use of an expensive and elaborate 
instrument, an observation takes considerable time, 
and the values of the principal refractive indices 
cannot in general be immediately determined. 

Table III at the end of the book gives the 
refractive indices of the gem-stones. 



Prism-angle 




CHAPTER V 
LUSTRE AND SHEEN 

IT has been already stated that whenever light in 
one medium falls upon the surface separating 
it from another medium some of the light is 
reflected within the first, while the remainder passes 
out into the second medium, except when the first 
is of lower refractivity than the second and light 
falls at an angle greater than that of total-reflection. 
Similarly, when light impinges upon a cut stone 
some of it is reflected and the remainder passes into 
the stone. What is the relative amount of reflected 
light depends upon the nature of the stone its 
refractivity and hardness and determines its 
lustre ; the greater the amount the more lustrous 
will the stone appear. There are different kinds of 
lustre, and the intensity of each depends on the 
polish of the surface. From a dull, i.e. an uneven, 
surface the reflected light is scattered, and there are 
no brilliant reflections. All gem-stones take a good 
polish, and have therefore, so long as the surface 
retains its polish, considerable brilliancy; turquoise, on 
account of its softness, is always comparatively dull. 
The different kinds of lustre are 

(1) Adamantine, characteristic of diamond. 

(2) Vitreous, as seen on the surface of 

fractured glass. 

(3) Resinous, as shown by resins. 



38 GEM-STONES 

Zircon and demantoid, the green garnet called by 
jewellers " olivine," alone among gem-stones have a 
lustre approaching that of diamond. The remainder 
all have a vitreous lustre, though varying in degree, 
the harder and the more refractive species being on 
the whole the more lustrous. 

Some stones for instance, a cinnamon garnet 
appear to have a certain greasiness in the lustre, 
which is caused by stray reflections from inclusions 
or other breaks in the homogeneity of the interior. 
A pearly lustre, which arises from cleavage cracks 
and is typically displayed by the cleavage face of 
topaz, would be seen in a cut stone only when 
flawed. 

Certain corundums when viewed in the direction 
of the crystallographical axis display six narrow 
lines of light radiating at angles of 60 from a 
centre in a manner suggestive of the conventional 
representations of stars. Such stones are con- 
sequently known as asterias, or more usually star- 
stones star-rubies or star-sapphires, as the case 
may be, and the phenomenon is called asterism. 
These stones have not a homogeneous structure, 
but contain tube-like cavities regularly arranged 
at angles of 60* in planes at right angles to the 
crystallographical axis. The effect is best produced 
when the stones are cut en cabochon perpendicular 
to that axis. 

Chatoyancy is a somewhat similar phenomenon, 
but in this case the fibres or cavities are parallel 
to a single direction, and a single broadish band 
is displayed at right angles to it. Cat's-eyes, as 
these stones are termed, are cut en cabochon parallel to 
the fibres. The true cat's-eye (Plate XXIX, Fig. i) 



LUSTRE AND SHEEN 39 

is a variety of chrysoberyl, but the term is also 
often applied to quartz showing a similar appearance. 
The latter is really a fibrous mineral, such as 
asbestos, which has become converted into silica. 
The beautiiul tiger's-eye from South Africa is a 
silicified crocidolite, the original blue colour of which 
has been altered by oxidation to golden brown. 
Recently tourmalines have been discovered which 
are sufficiently fibrous in structure to display an 
effective chatoyancy. 

The milky sheen of moonstone (Plate XXIX, 
Fig. 4) owes its effect to reflections from twin 
lamellae. The wonderful iridescence which is the 
glory of opal, and is therefore termed opalescence, 
arises from a structure which is peculiar to that 
species. Opal is a solidified jelly ; on cooling it 
has become riddled with extremely thin cracks, 
which were subsequently filled with similar material 
of slightly different refractivity, and thus it consists 
of a series of films. At the surface of each film 
interference of light takes place just as at the surface 
of a soap-bubble, and the more evenly the films are 
spaced apart the more uniform is the colour displayed, 
the actual tint depending upon the thickness of the 
films traversed by the light giving rise to the 
phenomenon. 



CHAPTER VI 
DOUBLE REFRACTION 

r I ^HE optical phenomenon presented by many 
J. gem-stones is complicated by their property 
of splitting up a beam of light into two with, in 
general, differing characters. In this chapter we 
shall discuss the nature of double refraction, as it is 
termed, and methods for its detection. The pheno- 
menon is not one that comes within the purview of 
everyday experience. 

So long ago as 1669 a Danish physician, by 
name Bartholinus, noticed that a plate of the trans- 
parent mineral which at that time had recently been 
brought over from Iceland, and was therefore called 
" Iceland-spar," possessed the remarkable property 
of giving a double image of objects close to it when 
viewed through it. Subsequent investigation has 
shown that much crystallized matter is doubly 
refractive, but in calcite to use the scientific name 
for the species which includes Iceland-spar alone 
among common minerals is the phenomenon so 
conspicuous as to be obvious to the unaided eye. 
The apparent separation of the pair of images given 
by a plate cut or cleaved in any direction depends 
upon its thickness. The large mass, upwards of 
two feet (60 cm.) in thickness, which is exhibited 
at the far end of the Mineral Gallery of the British 



DOUBLE REFRACTION 41 

Museum (Natural History), displays the separation 
to a degree that is probably unique. 

Although none of the gem-stones can emulate 
calcite in this character, yet the double refraction 
of certain of them is large enough to be detected 
without much difficulty. In the case of faceted 
stones the opposite edges should be viewed through 
the table-facet, and any signs of doubling noted. 




FIG. 24. Apparent doubling of the Edges of a Peridot when 
viewed through the Table-Facet. 

The double refraction of sphene is so large, viz. 
O'O8, that the doubling of the edges is evident to 
the unaided eye. In peridot (Fig. 24), zircon (b), 
and epidote the apparent separation of the edges is 
easily discerned with the assistance of an ordinary 
lens. A keen eye can detect the phenomenon even 
in the case of such substances as quartz with small 
double refraction. It must, however, be remembered 
that in all such stones the refraction is single in 
certain directions, and the amount of double refraction 



42 GEM-STONES 

varies therefore with the direction from nil to the 
maximum possessed by the stone. Experiment 
with a plate of Iceland-spar shows that the rays 
transmitted by it have properties differing from 
those of ordinary light On superposing a second 
plate we notice that there are now two pairs of 
images, which are in general no longer of equal 
brightness, as was the case before. If the second 
plate be rotated with respect to the first, two images, 
one of each pair, disappear, and then the other two, 
the plate having turned through a right angle 
between the two positions of extinction ; midway 
between these positions the images are all equally 




a 
FIG. 25. Wave-Motion. 

bright This variation of intensity implies that 
each of the rays emerging from the first plate has 
acquired a one-sided character, or, as it is usually 
expressed, has become plane-polarized, or, shortly, 
polarized. 

Before the discovery of the phenomenon of double 
refraction the foundation of the modern theory of 
light had been laid by the genius of Huygens. 
According to this theory light is the result of a 
wave-motion (Fig. 25) in the ether, a medium that 
pervades the whole of space whether occupied by 
matter or not, and transmits the wave-motion at a 
rate varying with the matter with which it happens 
to coincide. Such a medium has been assumed 



DOUBLE REFRACTION 43 

because it explains satisfactorily all the phenomena 
of light, but it by no means follows that it has a 
concrete existence. Indeed, if it has, it is so 
tenuous as to be imperceptible to the most delicate 
experiments. The wave-motion is similar to that 
observed on the surface of still water when disturbed 
by a stone flung into it. The waves spread out 
from the source of disturbance; but, although the 
waves seem to advance, the actual particles of water 
merely move up and down, and have no motion at 
all in the direction in which the waves are moving. 
If we imagine similar motion to take place in any 
plane and not only the horizontal, we form some idea 
of the nature of ordinary light. But after passing 
through a plate of Iceland-spar, light no longer 
vibrates in all directions, but in each beam the 
vibrations are parallel to a particular plane, the two 
planes being at right angles. The exact relation of 
the direction of the vibrations to the plane of polariz- 
ation is uncertain, although it undoubtedly lies in the 
plane containing the direction of the ray of light and 
the perpendicular to the plane of polarization. The 
waves for different colours differ in their length, i.e. 
in the distance, 2 bb (Fig. 25), from crest to crest, 
while the velocity, which remains the same for the 
same medium, is proportional to the wave-length. 
The intensity of the light varies as the square of the 
amplitude of the wave, i.e. the height, ab, of the 
crest from the mean level. 

Various methods have been proposed for obtain- 
ing polarized light. Thus Seebeck found in 1813 
that a plate of brown tourmaline cut parallel to the 
crystallographic axis and of sufficient thickness 
(cf. p. n) transmits only one ray, the other being 



44 GEM-STONES 

entirely absorbed within the plate. Another method 
was to employ a glass plate to reflect light at a 
certain critical angle. The most efficient method, 
and that in general use at the present day, is due 
to the invention of Nicol. A rhomb of Iceland- 
spar (Fig. 26), of suitable length, is sliced along the 
longer diagonal, dd, and the halves are cemented 
together by means of Canada balsam. One ray, 
ioo, is totally reflected at the surface separating the 
mineral and the cement, and does not penetrate 
into the other half; while the other ray, iee, is trans- 
mitted with almost undiminished intensity. Such 




FIG. 26. Nicol's Prism. 

a rhomb is called a Nicol's prism after its inventor, 
or briefly, a nicol. 

If one nicol be placed above another and their 
corresponding principal planes be at right angles 
no light is transmitted through the pair. In the 
polarizing microscope one such nicol, called the 
polarizer, is placed below the stage, and the other, 
called the analyser, is either inserted in the body 
of the microscope or placed above the eyepiece, and 
the pair are usually set in the crossed position so 
that the field of the microscope is dark. If a piece 
of glass or a fragment of some singly refractive sub- 
stance be placed on the stage the field still remains 



DOUBLE REFRACTION 45 

dark ; but in case of a doubly refractive stone the 
field is no longer dark except in certain positions 
of the stone. On rotation of the plate, or, if 
possible, of the nicols together, the field passes from 
darkness to maximum brightness four times in a 
complete revolution, the relative angular intervals 
between these positions being right angles. These 
positions of darkness are known as the positions of 
extinction, and the plate is said to extinguish in 
them. This test is exceedingly delicate and reveals 
the double refraction even when the greatest 
difference in the refractive indices is too small to 
be measured directly. 

Doubly refractive substances are of two kinds: 
uniaxial, in which there is one direction of single 
refraction, and biaxial, in which there are two such 
directions. In the case of the former the direction 
of one, the ordinary ray, is precisely the same as if 
the refraction were single, but the refractive index 
of the other ray varies from that of the ordinary 
ray to a second limiting value, the extraordinary 
refractive index, which may be either greater or less. 
If the extraordinary is greater than the ordinary 
refractive index the double refraction is said to be 
positive ; if less, to be negative. A biaxial substance 
is more complex. It possesses three principal 
directions, viz., the bisectrices of the directions of 
single refraction and the perpendicular to the plane 
containing them. The first two correspond to the 
greatest and least, and the last to the mean of the 
principal indices of refraction. If the acute 
bisectrix corresponds to the least refractive index, 
the double refraction is said to be positive, and if to 
the greatest, negative. The relation of the .direc- 



GEM-STONES 



tions of single refraction, s, to the three principal 
directions, a, b, c, is illustrated in Fig. 27 for the 
case of topaz, a positive mineral. The refractive 
indices of the rays traversing one -of the principal 
directions have the values corresponding to the 
other two. In the direction a we should measure 
the greatest and the mean of the principal refractive 
indices, in the direction b the greatest and the least, 
and in the direction c the mean and the least. The 
maximum amount of double refraction is there- 
fore in the direction b. 

In the examination of a 
faceted stone, of the most 
usual shape, the simplest 
method is to lay the large 
facet, called the table, on a 
-b glass slip and view the stone 
through the small parallel 
facet, the culet. Should the 

FIG. 27,-Relation of the latter not exist > * mav fre - 
two Directions of single quently happen that owing 
Refraction to the prin- t o internal reflection no light 
emerges through the steeply 
inclined facets. This difficulty 
is easily overcome by immersing the stone in some 
highly refracting oil. A glass plate held by hand 
over the stone with a drop of the oil between it 
and the plate serves the purpose, and is perhaps a 
more convenient method. A stone which does not 
possess a pair of parallel facets should be viewed 
through any pair which are nearly parallel. 

We have stated that a plate of glass has no effect 
on the field. Suppose, however, it were viewed 
when placed between the jaws of a tightened vice 




DOUBLE REFRACTION 47 

and thus thrown into a state of strain, it would then 
show double refraction, the amount of which would 
depend on the strain. Natural singly refractive 
substances frequently show phenomena of a similar 
kind. Thus diamond sometimes contains a drop 
of liquid carbonic acid, and the strain is revealed 
by the coloured rings surrounding the cavity which 
are seen when the stone is viewed between crossed 
nicols. Double refraction is also common in 
diamond even when there is no included matter to 
explain it, and is caused by the state of strain into 
which the mineral is thrown on release from the 
enormous pressure under which it was formed. 
Other minerals which display these so-called optical 
anomalies, such as fluor and garnet, are not really 
quite singly refractive at ordinary temperatures ; 
each crystal is composed of several double refractive 
individuals. But all such phenomena cannot be 
confused with the characters of minerals which ex- 
tinguish in the ordinary way, since the stone will 
extinguish in small patches and these will not be 
dark all at the same time ; further, the double re- 
fraction is small, and on revolving the stone between 
crossed nicols the extinction is not sharp. Paste 
stones are sometimes in a state of strain, and 
display slight, but general, double refraction. 
Hence the existence of double refraction does not 
necessarily prove that the stone is real and not an 
imitation. Stones may be composed of two or 
more individuals which are related to each other 
by twinning, in which case each individual would 
in general extinguish separately. Such individuals 
would be larger and would extinguish more sharply 
than the patches of an anomalous stone. 



4 8 



GEM-STONES 



An examination in convergent light is sometimes 
of service. An auxiliary lens is placed over the 
condenser so as to converge the light on to the stone. 
Light now traverses the stone in different directions ; 
the more oblique the direction the greater the 
distance traversed in the stone. If it be doubly 
refractive, in any given direction there will be in 
general two rays with differing refractive indices and 
the resulting effect is akin to the well-known 

phenomenon of New- 
ton's rings, and is an 
instance of what is 
termed interference. 
It may be mentioned 
that the interference 
of light (Fig. 28) 
explains such com- 
mon phenomena as 
the colours of a 
soap-bubble, the hues 

of tarnished steel, the tints of a layer of oil floating 
on water, and so on. Light, after diverging from 
the stone, comes to focus a little beneath the plane 
in which the image of the stone is formed. An 
auxiliary lens must, therefore, be inserted to bring 
the focal planes together, so that the interference 
picture may be viewed by means of the same eye- 
piece. 

If a uniaxial crystal be examined along the 
crystallographic axis in convergent light an inter- 
ference picture will be seen of the kind illustrated on 
Plate III. The arms of a black cross meet in the 
centre of the field, which is surrounded by a series 
of circular rings, coloured in white light. Rotation 




FIG. 28. Interference of Light. 





I. UNIAXIAL 





INTERFERENCE FIGURES 



DOUBLE REFRACTION 49 

of the stone about the axis produces no change in 
the picture. 

A biaxial substance possesses two directions (the 
optic axes] along which a single beam is transmitted. 
If such a stone be examined along the line bisecting 
the acute angle between the optic axes (the acute 
bisectrix] an interference picture l will be seen which 
in particular positions of the stone with respect to 
the crossed nicols takes the forms illustrated on 
Plate III. As before, there is a series of rings 
which are coloured in white light ; they, however, 
are no longer circles but consist of curves known 
as lemniscates, of which the figure of 8 is a special 
form. Instead of an unchangeable cross there are 
a pair of black " brushes " which in one position of 
the stone are hyperbolae, and in that at right angles 
become a cross. On rotating the stone we find 
that the rings move with it and are unaltered in 
form, whereas the brushes revolve about two points, 
called the " eyes," where the optic axes emerge. If 
the observation were made along the obtuse bisectrix 
the angle between the optic axes would probably 
be too large for the brushes to come into the field, 
and the rings might not be visible in white light, 
though they would appear in monochromatic light. 
In the case of a substance like sphene the figure is 
not so simple, because the positions of the optic 
axes vary greatly for the different colours and the 
result is exceedingly complex ; in monochromatic 
light, however, the usual figure is visible. 

It would probably not be possible in the case of 

1 A cleavage flake of topaz may conveniently be used to show the 
phenomenon, but owing to the great width of the angle the "eyes" 
are invisible. 



50 GEM-STONES 

a faceted stone to find a pair of faces perpendicular 
to the required direction. Nevertheless, so long as 
a portion of the figures described is in the field of 
view, the character of the double refraction, whether 
uniaxial or biaxial, may readily be determined. 

There is yet another remarkable phenomenon 
which must not be passed over. Certain substances, 
of which quartz is a conspicuous example and in 
this respect unique among the gem-stones, possess 
the remarkable property of rotating the plane of 
polarization of a ray of light which is transmitted 
parallel to the optic axis. If a plate of quartz be 
cut at right angles to the axis and placed between 
crossed nicols in white light, the field will be 
coloured, the hue changing on rotation of one nicol 
with respect to the other. Examination in 
monochromatic light shows that the field will 
become dark after a certain rotation of the one 
nicol with respect to the other, the amount of which 
depends on the thickness of the plate. If the plate 
be viewed in convergent light, an interference picture 
is seen as illustrated on Plate III, which is similar to, 
and yet differs in some important particulars from 
the ordinary interference picture of a uniaxial stone. 
The cross does not penetrate beyond the innermost 
ring and the centre of the field is coloured in white 
light. If a stone shows such a picture, it may be 
safely assumed to be quartz. It is interesting to 
note that minerals which possess this property have 
a spiral arrangement of the constituent atoms. 

It has already been remarked (p. 28) that if a 
faceted doubly refractive stone be rotated with one 
facet always in contact with the dense glass of the 
refractometer the pair of shadow-edges that are 



DOUBLE REFRACTION 51 

visible in the field move up or down the scale in 
general from or to maximum and minimum 
positions. The manner in which this movement 
takes place depends upon the character of the 
double refraction and the position of the facet under 
observation with regard to the optical symmetry of 
the stone. In the case of a uniaxial stone, if the 
facet be perpendicular to the crystallographic axis, 
i.e. the direction of single refraction, neither of the 
shadow-edges will move. If the facet be parallel 
to that direction, one shadow-edge will move up and 
coincide with the other, which remains invariable 
in position, and away from it to a second critical 
position ; the latter gives the value of the extra- 
ordinary refractive index, and the invariable shadow- 
edge corresponds to the ordinary refractive index. 
This phenomenon is displayed by the table-facet 01 
most tourmalines, because for reasons given above 
(p. 11) they are as a rule cut parallel to the 
crystallographic axis. In the case of facets in 
intermediate positions, the shadow-edge correspond- 
ing to the extraordinary refractive index moves, but 
not to coincidence with the invariable shadow-edge. 
The case of a biaxial stone is more complex. If 
the facet be perpendicular to one of the principal 
directions one shadow-edge remains invariable in 
position, corresponding to one of the principal 
refractive indices, whilst the other moves between 
the critical values corresponding to the remaining 
two of the principal refractive indices. In the 
interesting case in which the facet is parallel to the 
two directions of single refraction, the second shadow- 
edge moves across the one which is invariable in 
position. In intermediate positions of the facet both 



52 GEM-STONES 

shadow-edges move, and give therefore critical values. 
Of the intermediate pair, i.e. the lower maximum and 
the higher minimum, one corresponds to the mean 
principal refractive index, and the other depends 
upon the relation of the facet to the optical 
symmetry. If it is desired to distinguish between 
them, observations must be made on a second facet ; 
but for discriminative purposes such exactitude is 
unnecessary, since the least and the greatest refractive 
indices are all that are required. 

The character of the refraction of gem-stones is 
given in Table V at the end of the book. 



CHAPTER VII 

ABSORPTION EFFECTS: COLOUR, DICHROISM, 
ETC. 

WHEN white light passes through a cut stone, 
colour effects result which arise from a 
variety of causes. The most obvious is the funda- 
mental colour of the stone, which is due to its 
selective absorption of the light passing through it, 
and would characterize it before it was cut. Inter- 
mingled with the colour in a transparent stone is 
the dispersive effect known as 'fire,' which has 
already been discussed (p. 20). In many instances 
the want of homogeneity is responsible for some 
peculiar effects such as opalescence, chatoyancy, and 
asterism. These phenomena will now be considered 
in fuller detail. 

COLOUR 

All substances absorb light to some extent. If 
the action is slight and affects equally the whole of 
the visible spectrum, the stone appears white or 
colourless. Usually some portion is more strongly 
absorbed than the rest, and the stone seems to be 
coloured. What is the precise tint depends not 
only upon the portions transmitted through the 
stone, but also upon their relative intensities. The 
eye, unlike the ear, has not the power of analysis 



54 GEM-STONES 

and it cannot of itself determine how a composite 
colour has been made up. Indeed, so far as it is 
concerned, any colour may be exactly matched by 
compounding in certain proportions three simple 
primary colours red, yellow, and violet. Alex- 
andrite, a variety of chrysoberyl, is a curious and 
instructive case. The balance in the spectrum of 
light transmitted through it is such that, whereas in 
daylight such stones appear green, in artificial light, 
especially in gas-light, they are a pronounced 
raspberry-red (Plate XXVII, Figs, n, 13). The 
phenomenon is intensified by the strong dichroism 
characteristic of this species. 

The colour is the least reliable character that may 
be employed for the identification of a stone, since it 
varies considerably in the same species, and often 
results from the admixture of some metallic oxide, 
which has no essential part in the chemical com- 
position and is present in such minute quantities 
as to be almost imperceptible by analysis. Who 
would, for instance, imagine from their appearance 
that stones so markedly diverse in hue as ruby 
and sapphire were really varieties of the same species, 
corundum ? Again, quartz, in spite of the simplicity 
of its composition, displays extreme differences of 
tint. Nevertheless, certain varieties do possess a 
distinctive colour, emerald being the most striking 
example, and in other cases the trained eye can 
appreciate certain characteristic subtleties of shade. 
At any rate, the colour is the most obvious of the 
physical characters, and serves to provide a rough 
division of the species, and accordingly in Table II 
at the end of the book the gem-stones are arranged 
by their usual tints. 



ABSORPTION EFFECTS 



55 



DlCHROISM 

The two rays into which a doubly refractive stone 
splits up a ray of light are often differently absorbed 
by it, and in consequence appear on emergence 
differently coloured ; such stones are said to be 
dichroic. The most striking instance is a deep- 
brown tourmaline, which, except in very thin 
sections, is quite opaque to the ordinary ray. The 
light transmitted by a plate cut parallel to the 




FIG. 29. Dichroscope (actual size). 

crystallographic axis is therefore plane-polarized ; 
before the invention by Nicol of the prism of Iceland- 
spar known by his name this was the ordinary 
method of obtaining light of this character (cf. p. 43). 
Again, in the case of kunzite and cordierite the 
difference in colour is so marked as to be obvious to 
the unaided eye ; but where the contrast is less 
pronounced we require the use of an instrument 
called a dichroscope, which enables the twin colours 
to be seen side by side. 

Fig. 29 illustrates in section the construction of a 
dichroscope. The instrument consists essentially of 



5 6 GEM-STONES 

a rhomb of Iceland-spar, S, of such a length as to 
give two contiguous images (Fig. 30) of a square 
hole, //, in one end of the tube containing it. In 
some instruments the terminal faces of the rhomb 
are ground at right angles to its 
length, but usually, as in that 
depicted, prisms of glass, G, are 
cemented on to the two ends. A 

CE P C > with a sli htl y lar er h le > 




FIG. 30. -Field of ...... , ^ 

the Dichroscope. which is circular in shape, fits on 
the end of the tube, and can be 
moved up and down it and revolved round it, as 
desired. The stone, R, to be tested may be 
directly attached to it by means of some kind of 
wax or cement in such a way that light which has 
traversed it passes into the window, H, of the in- 
strument ; the cap at the same time permits of 
the rotation of the stone about the axis of the main 
tube of the instrument. The dichroscope shown in 
the figure has a still more convenient arrangement : 
it is provided with an additional attachment, A, by 
means of which the stone can be turned about an 
axis at right angles to the length of the tube, and 
thus examined in different directions. At the other 
end of the main tube is placed a lens, L, of low 
power for viewing the twin images : the short tube 
containing it can be pushed in and out for focusing 
purposes. Many makers now place the rhomb close 
to the lens, L, and thereby require a much smaller 
piece of spar ; material suitable for optical purposes 
is fast growing scarce. 

Suppose that a plate of tourmaline cut parallel to 
its crystallographic axis is fastened to the cap and 
the latter rotated. We should notice, on looking 



ABSORPTION EFFECTS 57 

through the instrument, that in the course of a 
complete revolution there are two positions, ori- 
entated at right angles to one another, in which 
the tints of the two images are identical, the 
positions of greatest contrast of tint being midway 
between. If we examine a uniaxial stone in a 
direction at right angles to its optic axis we obtain 
the colours corresponding to the ordinary and the 
extraordinary rays. In any direction less inclined 
to the axis we still have the colour for the ordinary 
ray, but the other colour is intermediate in tint 
between it and that for the extraordinary ray. The 
phenomenon presented by a biaxial stone is more 
complex. There are three principal colours which 
are visible in differing pairs in the three principal 
optical directions ; in other directions the tints seen 
are intermediate between the principal colours. 
Since biaxial stones have three principal colours, 
they are sometimes said to be trichroic or pleochroic, 
but in any single direction they have two twin 
colours and show dichroism. No difference at all 
will be shown in directions in which a stone is 
singly refractive, and it is therefore always advisable 
to examine a stone in more than one direction lest 
the first happens to be one of single refraction. For 
determinative purposes it is __not ijiecessary to note 
the exact shades of tint of the twin colours, because 
they vary with the inherent colour of the stone, and 
are therefore not constant for the same species ; we 
need only observe, when the stone is tested with the 
dichroscope, whether there is any variation of colour, 
and, if so, its strength. Dichroism is a result of 
double refraction, and cannot exist in a singly 
refractive stone. The converse, however, is not true 



58 GEM-STONES 

and it by no means follows that, because no dichroism 
can be detected in a stone, it is singly refractive. A 
colourless stone, for instance, cannot possibly be 
dichroic, and many coloured, doubly refractive 
stones for example, zircon exhibit no dichroism, or 
so little that it is imperceptible. The character is 
always the better displayed, the deeper the inherent 
colour of the stone. The deep-green alexandrite, 
for instance, is far more dichroic than the lighter 
coloured varieties of chrysoberyl. 

If the stone is attached to the cap of the 
instrument, the table should be turned towards it so 
as to assure that the light passing into the instru- 
ment has actually traversed the stone. If little 
light enters through the opposite coign, a drop of oil 
placed thereon will overcome the difficulty (cf. p. 46). 
It is also necessary, for reasons mentioned above, to 
examine the stone in directions as far as possible 
across the girdle also. A convenient, though not 
strictly accurate, method is to lay the stone with the 
table facet on a table and examine the light which 
has entered the stone and been reflected at that 
facet. The stone may easily be rotated on the 
table, and observations thus made in different 
directions in the stone. Care must be exercised 
in the case of a faceted stone not to mistake the 
alteration in colour due to dispersion for a dichroic 
effect, and the stone must be placed close to the instru- 
ment during an observation, because otherwise the 
twin rays traversing the instrument may have taken 
sensibly different directions in the stone. 

Dichroism is an effective test in the case of ruby; 
its twin colours purplish and yellowish red are 
in marked contrast, and readily distinguish it from 



ABSORPTION EFFECTS 59 

other red stones. Again, one of the twin colours 
of sapphire is distinctly more yellowish than the 
other ; the blue spinel, of which a good many have 
been manufactured during recent years, is singly 
refractive, and, of course, shows no difference of tint 
in the dichroscope. 

Table VI at the end of the book gives the 
strength of the dichroism of the gem-stones. 

ABSORPTION SPECTRA 

A study of the chromatic character of the light 
transmitted by a coloured stone is of no little 
interest. As was stated above, the eye has not the 
power of analysing light, and to resolve the trans- 
mitted rays into their component parts an instru- 
ment known as a spectroscope is needed. The 
small ' direct-vision ' type has ample dispersion for 
this purpose. It is advantageous to employ by 
preference the diffraction rather than the prism 
form, because in the former the intervals in the 
resulting spectrum corresponding to equal differences 
of wave-length are the same, whereas in the latter 
they diminish as the wave-length increases and 
accordingly the red end of the spectrum is relatively 
cramped. 

The absorptive properties of all doubly refractive 
coloured substances vary more or less with the 
direction in which light traverses them according to 
the amount of dichroism that they possess, but the 
variation is not very noticeable unless the stone is 
highly dichroic. If the light transmitted by a deep- 
coloured ruby be examined with a spectroscope it 
will be found that the whole of the green portion 



6o 



GEM-STONES 



of the spectrum is obliterated (Fig. 31), while in the 
case of a sapphire only a small portion of the red 
end of the spectrum is absorbed. Alexandrite 
affords especial interest. In the spectrum of the 



ALMANDINE 



ALEXANDRITE 



RUBY 



THE SOLAR SPECTRUM 
FIG. 31. Absorption Spectra. 

light transmitted by it, the violet and the yellow are 
more or less strongly absorbed, depending upon the 
direction in which the rays have passed through the 
stone (Fig. 31), and the transmitted light is mainly 
composed of two portions red and green. The 
apparent colour of the stone depends, therefore, upon 



ABSORPTION EFFECTS 61 

which of the two predominates. In daylight the 
resultant colour is green flecked with red and orange, 
the three principal absorptive tints (cf. p. 235), but in 
artificial light, which is relatively stronger in the red 
portion of the spectrum, the resultant colour is a 
raspberry-red, and there is less apparent difference in 
the absorptive tints (cf. Plate XXVII, Figs. 1 1, 13). 

In all the spectra just considered, and in all like 
them, the portions that are absorbed are wide, the 
passage from blackness to colour is gradual, and 
the edges deliminating them are blurred. In the 
spectra of certain zircons and in almandine garnet 
the absorbed portions, or bands as they are called, 
are narrow, and, moreover, the transition from black- 
ness to colour is sharp and abrupt ; such stones are 
therefore said to display absorption-bands. Church 
in 1866 was the first to notice the bands shown by 
zircon (Fig. 31). Sorby thought they portended the 
existence of a new element, to which he gave the 
name jargonium, but subsequently discovered that 
they were caused by the presence of a minute trace 
of uranium. A yellowish-green zircon shows the 
phenomenon best, and it has all the bands shown 
in the figure. The spectrum varies slightly but 
almost imperceptibly with the direction in the stone. 
Others show the bands in the yellow and green, 
while others show only those in the red, and some 
only one of them. The bands are not confined to 
stones of any particular colour, or amount of double 
refraction. Again, many zircons show no bands at 
all, so that their absence by no means precludes the 
stone from being a zircon. 

Almandine is characterized by a different spectrum 
(Fig. 31). The band in the yellow is the most con- 



62 GEM-STONES 

spicuous, and is no doubt responsible for the purple 
hue of a typical almandine. The spectrum varies 
in strength in different stones. Rhodolite (p. 2 14), a 
garnet lying between almandine and pyrope, displays 
the same bands, and indications of them may be 
detected in the spectra of pyropes of high refraction. 




JEWELLERY DESIGNS 



64 GEM-STONES 

species, and is therefore very useful for discrimina- 
tive purposes. It can be determined whatever be 
the shape of the stone, and it is immaterial whether 
it be transparent or not ; but, on the other hand, the 
stone must be unmounted and free from the setting. 
The methods for the determination of the specific 
gravity are of two kinds : in the first a liquid is 
found of the same, or nearly the same, density as 
the stone, and in the second weighings are made 
and the use of an accurate balance is required. 

(i) HEAVY LIQUIDS 

Experiment tells us that a solid substance floats in 
a liquid denser than itself, sinks in one less dense, 
and remains suspended at any level in one of pre- 
cisely the same density. If the stone be only 
slightly less dense than the liquid, it will rise to the 
surface ; if it be just as slightly denser, it will as 
surely sink to the bottom, a physical fact which has 
added so much to the difficulty and danger of sub- 
marine manoeuvring. If then we can find a liquid 
denser than the stone to be tested, and place the 
latter in it, the stone will float on the surface. If we 
take a liquid which is less dense than the stone and 
capable of mixing with the heavier liquid, and add 
it to the latter, drop by drop, gently stirring so as 
to assure that the density of the combination is 
uniformly the same throughout, a stage is finally 
reached when the stone begins to move downwards. 
It has now very nearly the density of the liquid, 
and, if we find by some means this density, we 
know simultaneously the specific gravity of the 
stone. 



SPECIFIC GRAVITY 65 

Various devices and methods are available for 
ascertaining the density of liquids for instance, 
Westphal's balance ; but, apart from the incon- 
venience attending such a determination, the density 
of all liquids is somewhat seriously affected by 
changes in the temperature, and it is therefore better 
to make direct comparison with fragments of sub- 
stances of known specific gravity, which are termed 
indicators. If of two fragments differing slightly in 
specific gravity one floats on the surface of a uniform 
column of liquid and the other lies at the bottom of 
the tube containing the liquid, we may be certain 
that the density of the liquid is intermediate between 
the two specific gravities. Such a precaution is 
necessary because, if the liquid be a mixture of two 
distinct liquids, the density would tend to increase 
owing to the greater volatility of the lighter of them, 
and in any case the density is affected by change of 
temperature. The specific gravity of stones is not 
much altered by variation in the temperature. 

A more convenient variation of this method is to 
form a diffusion column, so that the density increases 
progressively with the depth. If the stone under 
test floats at a certain level in such a column inter- 
mediate between two fragments of known specific 
gravity, its specific gravity may be found by 
elementary interpolation. To form a column of 
this kind the lighter liquid should be poured on to 
the top of the heavier. Natural diffusion gives 
the most perfect column, but, being a lengthy 
process, it may conveniently be quickened by gently 
shaking the tube, and the column thus formed 
gives results sufficiently accurate for discriminative 
purposes. 
5 



66 GEM-STONES 

By far the most convenient liquid for ordinary 
use is methylene iodide, which has already been 
recommended for its high refraction. It has, when 
pure, a density at ordinary room-temperatures of 
3'324, and it is miscible in all proportions with 
benzol, whose density is cr88, or toluol, another 
hydrocarbon which is somewhat less volatile than 
benzol, and whose density is about the same, namely, 
O'86. When fresh, methylene iodide has only a 
slight tinge of yellow, but it rapidly darkens on 
exposure to light owing to the liberation of iodine 
which is in a colloidal form and cannot be removed 
by filtration, The liquid may, however, be easily 
cleared by shaking it up with any substance with 
which the iodine combines to form an iodide remov- 
able by filtration. Copper filings answer the purpose 
well, though rather slow in action ; mercury may 
also be used, but is not very satisfactory, because 
a small amount may be dissolved and afterwards be 
precipitated on to the stone under test, carrying it 
down to the bottom of the tube. Caustic potash 
(potassium hydroxide) is also recommended ; in this 
case the operation should preferably be carried out 
in a special apparatus which permits the clear liquid 
to be drawn off underneath, because water separates 
out and floats on the surface. In Fig. 32 three cut 
stones, a quartz (ft), a beryl (), and a tourmaline (c) 
are shown floating in a diffusion column of methy- 
lene iodide and benzol. Although the beryl is only 
slightly denser than the quartz, it floats at a 
perceptibly lower level. These three species are 
occasionally found as yellow stones of very similar 
tint. 

Various other liquids have been used or proposed 



SPECIFIC GRAVITY 



for the same purpose, of which two may be 
mentioned. The first of them is a saturated solu- 
tion of potassium iodide and mercuric iodide in 
water, which is known after the 
discoverer as Sonstadt's solution. 
It is a clear mobile liquid with an 
amber colour, having at 12 C. a 
density of 3*085 ; it may be mixed 
with water to any extent, and is 
easily concentrated by heating; 
moreover, it is durable and not sub- 
ject to alteration of any kind ; on 
the other hand, it is highly poisonous 
and cauterizes the skin, not being 
checked by albumen ; it also de- 



stroys brass-ware by amalgamating FlG> 32 ._ St ones 

the metal. The second is Klein's of different Spe- 

solution, a clear yellow liquid which cific Gravities 

has at 1 5' C. a density of 3-28. It S?2 

Consists of the boro - tungState Of of heavy Liquid. 

cadmium, of which the formula is 
9WO 3 .B 2 O 3 .2CdO.2H 2 O+i6Aq, dissolved in water, 
with which it may be diluted. If the salt be heated, 
it fuses at 75 C. in its own water of crystallization 
to a yellow liquid, very mobile, with a density of 
3-55. Klein's solution is harmless, but it cannot 
compare for convenience of manipulation with 
methylene iodide. 

The most convenient procedure is to have at 
hand three glass tubes, fitted with stoppers or corks, 
to contain liquids of different densities 

(a) Methylene iodide reduced to 27 ; using as 
indicators orthoclase 2*55, quartz 2*66, and beryl 
274. 



68 GEM-STONES 

() Methylene iodide reduced to 3-1 ; indicators, 
beryl 2*74 and tourmaline 3*10. 

(c) Methylene iodide, undiluted, 3'32. 

The pure liquid in the last tube should on no 
account be diluted ; but the density of the other 
two liquids may be varied slightly, either by adding 
benzol in order to lower it, or by allowing benzol, 
which has far greater volatility than methylene 
iodide, to evaporate, or by adding methylene iodide, 
in order to increase it. The density of the liquids 
may be ascertained approximately from the in- 
dicators. 

A glance at the table of specific gravities shows 
that as regards the gem-stones methylene iodide is 
restricted in its application, since it can be used to 
test only moonstone, quartz, beryl, tourmaline, and 
spodumene; opal and turquoise, being amorphous 
and more or less porous, should not be immersed 
in liquids, lest the appearance of the stone be irre- 
trievably injured. Methylene iodide readily serves 
to distinguish the yellow quartz from the true topaz, 
with which jewellers often confuse it, the latter stone 
sinking in the liquid ; again aquamarine floats, but 
the blue topaz, which is often very similar to it, 
sinks in methylene iodide. 

By saturating methylene iodide with iodine and 
iodoform, we have a liquid (d} of density 3'6 ; a 
fragment of topaz, 3-55, may be used to indicate 
whether the liquid has the requisite density. Un- 
fortunately this saturated solution is so dark as to 
be almost opaque, and is, moreover, very viscous. 
Its principal use is to distinguish diamond, 3'535, 
from the brilliant colourless zircon, with which, 
apart from a test for hardness, it may easily be 



SPECIFIC GRAVITY 69 

confused. It is easy to see whether the stone 
floats, as it would do if a diamond. To recover a 
stone which has sunk, the only course is to pour 
off the liquid into another tube, because it is far 
too dark for the position of the stone to be seen. 

It is possible to employ a similar method for 
still denser stones by having recourse to Retgers's 
salt, silver-thallium nitrate. This double salt is 
solid at ordinary room-temperatures, but has the 
remarkable property of melting at a temperature, 
75 C., which is well below the point of fusion of 
either of its constituents, to a clear, mobile yellow 
liquid, which is miscible in any proportion with 
water, and has, when pure, a density of 4/6. The 
salt may be purchased, or it may be prepared by 
mixing 100 grams of thallium nitrate and 64 grams 
of silver nitrate, or similar proportions, in a little 
water, and heating the whole over a water-bath, 
keeping it constantly stirred with a glass rod until 
it is liquefied. The two salts must be mixed in the 
correct proportions, because otherwise the mixture 
might form other double salts, which do not melt 
at so low a temperature. A glance at the table of 
specific gravities shows that Retgers's salt may be 
used for all the gem-stones with the single exception 
of zircon (b). There are, however, some objections 
to its use. It is expensive, and, unless kept con- 
stantly melted, it is not immediately available. It 
darkens on exposure to strong sunlight like all 
silver salts, stains the skin a peculiar shade of 
purple which is with difficulty removed, and in fact 
only by abrasion of the skin, and, like all thallium 
compounds, is highly poisonous. 

It is convenient to have three tubes, fitted as 



70 GEM-STONES 

before with stoppers or corks, to contain the follow- 
ing liquids, when heated : 

(e) Silver-thallium nitrate, reduced to 3'5 ; using 

as indicators, peridot or idocrase 3-40 and topaz 

3*53. 

(/) Silver-thallium nitrate, reduced to 4-0; in- 
dicators, topaz 3-53 and sapphire 4-03. 

(g) Silver-thallium nitrate, undiluted, 4*6. 

The tubes must be heated in some form of water- 
bath ; an ordinary glass beaker serves the purpose 
satisfactorily. The pure salt should never be 
diluted; but the density of the contents of tubes 
(e) and (/) may be varied at will, water being 
added in order to lower the density, and concentra- 
tion by means of evaporation or addition of the 
nitrate being employed in order to increase it. To 
avoid the discoloration of the skin, rubber finger- 
stalls may be used, and the stones should not be 
handled until after they have been washed in warm 
water. The staining may be minimized if the 
hands be well washed in hot water before being 
exposed to sunlight. It is advisable to warm the 
stone to be tested in a tube containing water be- 
forehand lest the sudden heating develop cracks. 
A piece of platinum, or, failing that, copper wire is 
of service for removing stones from the tubes ; a 
glass rod, spoon-shaped at one end, does equally 
well. It must be noted that although Retgers's 
salt is absolutely harmless to the ordinary gem- 
stones with the exception of opal and turquoise, 
which, as has already been stated, being to some 
extent porous, should not be immersed in liquids 
it attacks certain substances, for instance, sulphides 
and cannot be applied indiscriminately to minerals. 



SPECIFIC GRAVITY 71 

The procedure described above is intended only 
as a suggestion ; the method may be varied to any 
extent at will, depending upon the particular re- 
quirements. If such tests are made only occasion- 
ally, a smaller number of tubes may be used. Thus 
one tube may be substituted for the two marked 
a and b, the liquid contained in it being diluted as 
required, and a series of indicators may be kept 
apart in small glass tubes. On the other hand, 
any one having constantly to test stones might in- 
crease the number of tubes with advantage, and 
might find it useful to have at hand fragments of 
all the principal species in order to make direct 
comparison. 

(2) DIRECT WEIGHING 

The balance which is necessary in both the 
methods described under this head should be 
capable of giving results accurate to milligrams, 
i.e. the thousandth part of a gram, and con- 
sistent with that restriction the beam may be as 
short as possible so as to give rapid swings and 
thus shorten the time taken in the observations. 
A good assay balance answers the purpose 
admirably. Of course, it is never necessary to 
wait till the balance has come to rest. The mean 
of the extreme readings of the pointer attached to 
the beam will give the position in which it would 
ultimately come to rest. Thus, if the pointer just 
touches the eighth division on the right-hand side 
and the second on the other, the mean position is 
the third division on the right-hand side (|(8 2) 
= 3). Instead of the ordinary form of chemical 
balance, Westphal's form or Joly's spring-balance 



72 GEM-STONES 

may be employed. Weighings are made more 
quickly, but are not so accurate. 

In refined physical work the practice known as 
double-weighing is employed to obviate any slight 
error there may be in the suspension of the balance. 
A counterpoise which is heavier than anything to 
be weighed is placed in one pan, and weighed. 
The counterpoise is retained in its pan throughout 
the whole course of the weighings. Any substance 
whose weight is to be found is placed in the other 
pan, and weights added till the balance swings 
truly again. The difference between the two sets 
of weights evidently gives the weight of the sub- 
stance. Balances, however, are so accurately con- 
structed that for testing purposes such refined 
precautions are not really necessary. 

It is immaterial in what notation the weighings 
are made, so long as the same is used throughout, 
but the metric system of weights, which is in 
universal use in scientific work, should preferably be 
employed. Jewellers, however, use carat weights, 
and a subdivision to the base 2 instead of decimals, 
the fractions being , -, , ^ J& -fa- If these 
weights be employed, it will be necessary to convert 
these fractions into decimals, and write | = '5oo, 

i = -250, i = -i 25, T V = -062, ^= -03 1, ^ = -016. 

(a) Hydrostatic Weighing 

The principle of this method is very simple. 
The stone, the specific gravity of which is required, 
is first weighed in air and then when immersed in 
water. If W and W be these weights respectively, 
then W W is evidently the weight of the water 



SPECIFIC GRAVITY 73 

displaced by the stone and having therefore the 
same volume as it, and the specific gravity is there- 

W 

fore equal to w _ w/ - 

If the method of double-weighing had been 
adopted, the formula would be slightly altered. 
Thus, suppose that c corresponds to the counter- 
poise, w and w' to the stone weighed in air and 
water respectively ; then we have W c w and 




FIG. 33. Hydrostatic Balance. 

W' = c w' y and therefore the specific gravity is 

c - w 
equal to -. . 

w - w 

Some precautions are necessary in practice to 
assure an accurate result A balance intended for 
specific gravity work is provided with an auxiliary 
pan (Fig. 33), which hangs high enough up to 
permit of the stone being suspended underneath. 
The weight of anything used for the suspension 
must, of course, be determined and subtracted from 
the weight found for the stone, both when in air 
and when in water. A piece of fine silk is generally 



74 GEM-STONES 

used for suspending the stone in water, but it should 
be avoided, because the water tends to creep up it 
and the error thus introduced affects the first place 
of decimals in the case of a one-carat stone, the 
value being too high. A piece of brass wire shaped 
into a cage is much to be preferred. If the same 
cage be habitually used, its weight in air and when 
immersed in water to the customary extent in such 
determinations should be found once for all. 

Care must also be taken to remove all air-bubbles 
which cling to the stone or the cage ; their presence 
would tend to make the value too low. The surface 
tension of water which makes it cling to the wire 
prevents the balance swinging freely, and renders 
it difficult to obtain a weighing correct to a 
milligram when the wire dips into water. This 
difficulty may be overcome by substituting a liquid 
such as toluol, which has a much smaller surface 
tension. 

As has been stated above, the density of water 
at 4 C. is taken as unity, and it is therefore 
necessary to multiply the values obtained by the 
density of the liquid, whatever it be, at the tempera- 
ture of the observation. In Table IX, at the end 
of the book, are given the densities of water and 
toluol at ordinary room-temperatures. It will be 
noticed that a correct reading of the temperature 
is far more important in the case of toluol. 

Example of a Hydrostatic Determination of 
Specific Gravity 

Weight of stone in air = I '47 1 gram 
Weight of stone in water = I '067 ,, 



SPECIFIC GRAVITY 75 

Allowing for the density of water at the tempera- 
ture of the room, which was 16 C., the specific 
gravity is 3'637. Had no such allowance been 
made, the result would have been four units too high 
in the third place of decimals. For discriminative 
purposes, however, such refinement is unnecessary. 

(b) Pycnometer^ or Specific Gravity Bottle 

The specific gravity bottle is merely one with 
a fairly long neck on which a horizontal mark has 
been scratched, and which is closed by a ground 
glass stopper. The pycnometer is a refined variety 
of the specific gravity bottle. It has two openings : 
the larger is intended for the insertion of the stone 
and the water, and is closed by a stopper through 
which a thermometer passes, while the other, 
which is exceedingly narrow, is closed by a stopper 
fitting on the outside, and is graduated to facilitate 
the determination of the height of the water in it. 

The stone is weighed as in the previous method. 
The bottle is then weighed, and filled with water 
up to the mark and weighed again. The stone is 
now introduced into the bottle, and the surplus 
water removed with blotting-paper or otherwise 
until it is at the same level as before, and the bottle 
with its contents is weighed. Let W be the weight 
of the stone, w the weight of the bottle, W the 
weight of the bottle and the water contained in it, 
and W" the weight of the bottle when containing 
the stone and the water. Then W -w is the 
weight of the water filling the bottle up to the 
mark, and W" w W is the reduced weight of 
water after the stone has been inserted ; the difference, 



76 GEM-STONES 

W+W- W" t is the weight of the water displaced. 

W 

The specific gravity is therefore - 5 . 

W + W W 

As in the previous method, this value must be 
multiplied by the density of the liquid at the 
temperature of the experiment. If the method 
of double-weighing be adopted, the formula will be 
slightly modified. 

Of the above methods, that of heavy liquids, as 
it is usually termed, is by far the quickest and the 
most convenient for stones of ordinary size, the 
specific gravity of which is less than the density of 
pure methylene iodide, namely, 3*324, and by its aid 
a value may be obtained which is accurate to the 
second place of decimals, a result quite sufficient 
for a discriminative test. The method is applicable 
no matter how small the stone may be, and, indeed, 
for very small stones it is the only trustworthy 
method ; for large stones it is inconvenient, not only 
because of the large quantity of liquid required, but 
also on account of the difficulty in estimating with 
sufficient certainty the position of the centre of 
gravity of the stone. A negative determination may 
be of value, especially if attention be paid to the rate 
at which the stone falls through the liquid ; the 
denser the stone the faster it will sink, but the rate 
depends also upon the shape of the stone. Retgers's 
salt is less convenient because of the delay involved 
in warming it and of the almost inevitable staining 
of the hands, but its use presents no difficulty 
whatever. 

Hydrostatic weighing is always available, unless 
the stone be very small, but the necessary weighings 



SPECIFIC GRAVITY 77 

occupy considerable time, and care must be taken 
that no error creeps into the computation, simple 
though it be. Even if everything is at hand, a 
determination is scarcely possible under a quarter 
of an hour. 

The third method, which takes even longer, is 
intended primarily for powdered substances, and is 
not recommended for cut stones, unless there happen 
to be a number of tiny ones which are known to 
be exactly of the same kind. 

The specific gravities of the gem-stones are given 
in Table VII at the end of the book. 



CHAPTER IX 
HARDNESS AND CLE A V ABILITY 

EVERY possessor of a diamond ring is aware 
that diamond easily scratches window-glass. 
If other stones were tried, it would be found that 
they also scratched glass, but not so readily, and, 
if the experiment were extended, it would be found 
that topaz scratches quartz, but is scratched by 
corundum, which in its turn yields to the all- 
powerful diamond. There is therefore considerable 
variation in the capacity of precious stones to 
resist abrasion, or, as it is usually termed, in their 
hardness. To simplify the mode of expressing this 
character the mineralogist Mohs about a century 
ago devised the following arbitrary scale, which is 
still in general use. 

MOHS'S SCALE OF HARDNESS 



i. Talc 
2. Gypsum 
3. Calcite 


4. Fluor 
5. Apatite 
6. Orthoclase 
10. Diamond 


7. Quartz 
8. Topaz 
9. Corundum 



A finger-nail scratches gypsum and softer sub- 
stances. Ordinary window-glass is slightly softer 
than orthoclase, and a steel knife is slightly harder ; 



HARDNESS AND CLEAV ABILITY 79 

a hardened file approaches quartz in hardness, and 
easily scratches glass. 

By saying that a stone has hardness 7 we merely 
mean that it will not scratch quartz, and quartz 
will not scratch it. The numbers indicate an order, 
and have no quantitative significance whatever. This 
is an important point about which mistakes are 
often made. We must not, for instance, suppose 
that diamond has twice the hardness of apatite. 
As a matter of fact, the interval between diamond 
and corundum is immensely greater than that 
between the latter and talc, the softest of mineral 
substances. Intermediate degrees of hardness 
are expressed by fractions. The number 8 for 
chrysoberyl means that it scratches topaz as easily 
as it itself is scratched by corundum. Pyrope 
garnet is slightly harder than quartz, and its 
hardness is said therefore to be "j\> 

Delicate tests show that the structure of all 
crystallized substances is more or less grained, like 
that of wood, and the hardness for the same stone 
varies in different directions. Kyanite is unique 
in this respect, since its hardness ranges from 5 to 
7 ; it can therefore be scratched by a knife in some 
directions, but not in others. In most substances, 
however, the range is so small as to be quite imper- 
ceptible. Slight variation is also apparent in the 
hardness of different specimens of the same species. 
The diamonds from Borneo and New South Wales 
are so distinctly harder than those from South 
Africa and other localities that, when first discovered, 
some difficulty was experienced in cutting them. 
Again, lapidaries find that while Ceylon sapphires 
are harder than rubies, Kashmir sapphires are softer. 



8o GEM-STONES 

Hardness is a character of fundamental importance 
in a stone intended for ornamental wear, since upon 
it depends the durability of the polish and brilliancy. 
Ordinary dust is largely composed of grains of 
sand, which is quartz in a minute form, and a 
gem-stone should therefore be at least as hard as 
that Paste imitations are little harder than 5, and 
consequently, as experience shows, their polish does 
not survive a few weeks' wear. Hardness is, 
however, of little use as a discriminative test except 
for distinguishing between topaz or harder stone and 
paste. Diamond is so much harder than other 
stones that it will leave a cut in glass quite different 
from the scratch of even corundum. Paste, being so 
soft, readily yields to the file, and is thus easily 
distinguished from genuine stones. In applying the 
test to a cut stone, it is best to remove it from its 
mount and try the effect on the girdle, because 
any scratch would be concealed afterwards by the 
setting. Any mark should be rubbed with the 
finger to assure that it is not due to powder from 
the scratching agent ; confusion may often be caused 
in this way when the two substances are of nearly 
the same hardness. 

The degrees of hardness of the gem-stones are 
given in Table VIII at the end of the book. 

It must not be overlooked that extreme hardness 
is compatible with cleavability in certain directions 
intimately connected with the crystalline structure ; 
the property, in fact, characterizes many mineral 
species of different degrees of hardness. Diamond 
can be split in four directions parallel to the faces of 
the regular octahedron, a property utilized by the 



HARDNESS AND CLEAVABILITY 81 

lapidary for shaping a stone previous to cutting it. 
Topaz cleaves with considerable ease at right angles 
to the principal crystallographic axis. Felspar has 
two directions of cleavage nearly at right angles to 
one another. The new gem-stone, kunzite, needs 
cautious handling owing to the facility with which 
it splits in two directions mutually inclined at 
about 70. 

All stones are more or less brittle, and will be 
fractured by a sufficiently violent blow, but the 
irregular surface of a fracture cannot be mistaken 
for the brilliant flat surface given by a cleavage. 
The cleavage is by no means induced with equal 
facility in the species mentioned above. A consider- 
able effort is required to split diamond, but in the 
case of topaz or kunzite incipient cleavage in the 
shape of flaws may be started if the stone be merely 
dropped on to a hard floor. 



CHAPTER X 
ELECTRICAL CHARACTERS 

THE definite orientation of the molecular 
arrangement of crystallized substances leads 
in many cases to attributes which vary with the 
direction and are revealed by the electrical properties. 
If a tourmaline crystal be heated in a gas or alcohol 
flame it becomes charged with electricity, and, since 
it is at the same time a bad conductor, static charges 
of opposite sign appear at the two ends. Topaz 
shows similar characters, but in a lesser degree. 
Quartz, if treated in the same way, shows charges 
of opposite sign on different sides, but the 
phenomenon may be masked by intimate twinning 
and consequent overlapping of the contrary areas. 
The phenomenon may also be seen when the 
stones are cut. The most convenient method for 
detecting the existence of the electrical charges is 
that devised by Kundt A powder consisting of a 
mixture of red lead and sulphur is placed in a 
bellows arrangement and blown through a sieve 
at one end on to the stone. Owing to the friction 
the particles become electrified red lead positively 
and sulphur negatively and are attracted by the 
charges of opposing sign, which will therefore be 
betrayed by the colour of the dust at the corre- 
sponding spot. The powder must be kept dry ; 



ELECTRICAL CHARACTERS 83 

otherwise a chemical reaction may occur leading to 
the formation of lead sulphide, recognizable by its 
black colour. Bucker has suggested as an alterna- 
tive the use of sulphur, coloured red with carmine, 
the negative element, and yellow lycopodium, the 
positive element. 

Diamond, topaz, and tourmaline are powerful 
enough, when electrified by friction with a cloth, to 
attract fragments of paper, the electrification being 
positive. Amber develops considerable negative 
electricity when treated in a similar manner. 

Diamond is translucent to the Rontgen (X) rays ; 
glass, on the other hand, is opaque to them, and 
this test distinguishes brilliants from paste imitations. 
Diamond also, unlike glass, phosphoresces under 
the influence of radium, a property characterizing 
also kunzite. 

It will be seen that the electrical characters, 
although of considerable interest to the student, 
are, on account of their limited application and 
difficulty of test, of little service for the discrimination 
of gem-stones. 



PART I SECTION B 

THE TECHNOLOGY OF GEM- 
STONES 

CHAPTER XI 
UNIT OF WEIGHT 

THE system in use for recording the weights of 
precious stones is peculiar to jewellery. 
The unit, which is known as the carat, bears no 
simple relation to any unit that has existed among 
European nations, and indubitably has been intro- 
duced from the East. When man in early days 
sought to record the weights of small objects, he 
made use of the most convenient seeds or grains 
which were easily obtainable and were at the same 
time nearly uniform in size. In Europe the 
smallest unit of weight was the barley grain. 
Similarly in the East the seeds of some leguminous 
tree were selected. Those of the locust-tree, Cera- 
tonia siliqua, which is common in the countries 
bordering the Mediterranean, on the average weigh 
so nearly a carat that they almost certainly formed 
the original unit. It is, indeed, from the Greek 
Kepdnov, little horn, which refers to the shape of the 
pods, that the word carat is derived. 



UNIT OF WEIGHT 85 

It is one of the eccentricities of the jewellery 
trade that precision should not have been given to 
the unit of weight. Not only does it vary at most 
of the trade centres in the world, but it is not even 
always constant at each centre. The difference 
is negligible in the case of single stones of 
ordinary size, but becomes a matter of serious 
importance when large stones, or parcels of small 
stones, are bought and sold, particularly when the 
stones are very costly. Attempts have been made 
at various times to secure a uniform standard, but 
as yet with only partial success. In 1871 the carat 
defined as the equivalent of 0*20500 gram was 
suggested at a meeting of the principal jewellers 
of Paris and London, and was eventually accepted 
in Paris, New York, Leipzig, and Borneo. It has, 
however, recently been recognized that in view of 
the gradual spread of the metric system of weights 
and measures the most satisfactory unit is the 
metric carat of one-fifth (0*2) gram. This has now 
been constituted the legal carat of France and 
Belgium, and no doubt other countries will follow 
their example. The carat weight obtaining in 
London weighs about 0-20530 gram, and the 
approximate equivalents in the gram at other 
centres are as follows: Florence 0-19720, Madrid 
0*20539, Berlin 0*20544, Amsterdam 0*20570, 
Lisbon 0*20575, Frankfort - on - Main 0*20577, 
Vienna 0-20613, Venice 0*20700, and Madras 
0*20735. The gram itself is inconveniently large 
to serve as a unit for the generality of stones met 
with in ordinary jewellery. 

The notation for expressing the sub-multiples 
of the carat forms another curious eccentricity. 



86 GEM-STONES 

Fractions are used which are powers of the half: 
thus the half, the half of that, i.e. the quarter, and 
so on down to the sixty-fourth, and the weight of 
a stone is expressed by a series of fractions, e.g. 
SaieV carats. In the case of diamond a single 
unreduced fraction to the base 64 is substituted 
in place of the series of single fractions, and the 
weight of a stone is stated thus, 4|- carats. With 
the introduction of the metric carat the more con- 
venient and rational decimal notation would, of 
course, be simultaneously adopted. 

Figs. 34-39 illustrate the exact sizes of diamonds 




10 carats. 
FIGS. 34-39. Exact Sizes of Brilliants of various Weights. 

of certain weights, when cut as brilliants. The 
sizes of other stones depends upon their specific 
gravity, the weight varying as the volume multiplied 
by the specific gravity. Quartz, for instance, has 
a low specific gravity and would be perceptibly 
larger, weight for weight; zircon, on the other 
hand, would be smaller. 

It has been found more convenient to select 
a smaller unit in the case of pearls, namely, the 
pearl-grain, four of which go to the carat. 

Stencil gauges are in use for measuring approxi- 
mately the weight in carats of diamond brilliants and 
of pearls, which in both instances must be unmounted. 
A more accurate method for determining the weight 



UNIT OF WEIGHT 87 

of diamonds has been devised by Charles Moe, which 
is applicable to either unmounted or mounted stones. 
By means of callipers, which read to three-tenths of 
a millimetre, the diameter and the depth of the stone 
are measured, and by reference to a table the corre- 
sponding weight is found ; allowance is made for 
the varying fineness of the girdle, and, in the case of 
large stones, for the variation from a strictly circular 
section. 



Since this chapter was written the movement in 
favour of the metric carat has made rapid progress, 
and this unit will soon have been adopted as the 
legal standard all over the world, even in countries, 
such as the British Isles and the United States, where 
the metric system is not in use. The advantage of 
an international unit is too obvious to need arguing. 



CHAPTER XII 
FASHIONING OF GEM-STONES 

ALTHOUGH many of the gem-stones have 
been endowed by nature with brilliant 
lustrous faces and display scintillating reflections 
from their surfaces, yet their form is never such as 
to reveal to full perfection the optical qualities upon 
which their charm depends. Moreover, the natural 
faces are seldom perfect ; as a rule the stones are 
broken either through some convulsion of the earth's 
crust or in course of extraction from the matrix in 
which they have lain, or they are roughened by 
attrition against matter of greater hardness, or worn 
by the prolonged action of water, or etched by 
solvents. Beautiful octahedra of diamond or spinel 
have been mounted without further embellishment, 
but even their appearance might have been much 
improved at the lapidary's hands. 

By far the oldest of the existing styles of cutting 
is the rounded shape known as cabochon, a French 
word derived from the Latin cabo, a head. In the 
days of the Roman Empire the softer stones were 
often treated in this manner ; such stones were 
supposed to be beneficial to those suffering from 
short-sightedness, the reason no doubt being that 
transparent stones when cut as a double cabochon 
formed a convex lens. According to Pliny, Nero 




JEWELLERY DESIGNS 



FASHIONING OF GEM-STONES 89 

had an emerald thus cut, through which he was 
accustomed to view the gladiatorial shows. This 
style of cutting was long a favourite for coloured 
stones, such as emerald, ruby, sapphire, and garnet, 
but has been abandoned in modern practice except 
for opaque, semi-opaque, and imperfect stones. 
The crimson garnet, which was at one time known 
by the name carbuncle, was so systematically thus cut 
that the word has come to signify a red garnet of 
this form. It was a popular brooch-stone with our 
grandmothers, but is no longer in vogue. The East 
still retains a taste for stones cut in the form of beads 
and drilled through the centre; the beads are 
threaded together, and worn as 
necklaces. The native lapidaries 
often improve the colour of pale 
emeralds by lining the hole with 

. / FIG. 40. Double 

green paint. (Convex) Ca . 

The cabochon form may be of bochon. 
three different kinds. In the first, 
the double cabochon (Fig. 40), both the upper and 
the under sides of the stones are curved. The 
curvature, however, need not be the same in each 
case; indeed, it is usually markedly different 
Moonstones and starstones are generally cut very 
steep above and shallow underneath. Occasion- 
ally a ruby or a sapphire is, when cut in this 
way, set with the shallow side above, because the 
light that has penetrated into the stone from above 
is more wholly reflected from a steep surface with 
consequent increase in the glow of colour from the 
stone. Opals are always cut higher on the exposed 
side, but the slope of the surface varies considerably ; 
they are generally cut steeply when required for 



90 GEM-STONES 

mounting in rings. Chrysoberyl cat's-eyes are 
invariably cut with curved bases in order to preserve 
the weight as great as possible. The double 
cabochon form with a shallow surface underneath 
merges into the second kind (Fig. 41) in which the 
under side is plane, the form commonly employed 
for quartz cat's-eyes, and occasion- 
ally also for carbuncles. In this type 

the plane side is invariably mounted 
FIG. 41. Simple r J 

Cabochon. down wards. In the third form 
(Fig. 42) the curvature of the under 
surface is reversed, and the stone is hollowed out 
into a concave shape. This style is reserved for 
dark stones, such as carbuncles, which, if cut at 
all thick, would show very little colour. A piece 
of foil is often placed in the hollow in order to 
increase the reflection of light, and 
thus to heighten the colour effect. .^^SS^^ 

In early days it was supposed that 

, , , f ,. FIG. 42. Double 

the extreme hardness of diamond (C oncavo - con- 
precluded the possibility of fashion- vex) Cabochon. 
ing it, and up to the fifteenth century 
all that was done was to remove the gum-like skin 
which disfigured the Indian stones and to polish the 
natural facets. The first notable advance was 
made in 1475, when Louis de Berquem discovered, 
as it is said quite by accident, that two diamonds if 
rubbed together ground each other. With confident 
courage he essayed the new art upon three large 
stones entrusted to him by Charles the Bold, to the 
entire satisfaction of his patron. The use of wheels 
or discs charged with diamond dust soon followed, 
but at first the lapidaries evinced their victory over 
such stubborn material by grinding diamond into 




FASHIONING OF GEM-STONES 91 

divers fantastic shapes, and failed to realize how 
much might be done to enhance the intrinsic beauty 
of the stones by the means now at their disposal 
The Indian lapidaries arrived at the same discovery 
independently, and Tavemier found, when visiting 
the country in 1665, a large number of diamond 
cutters actively employed. If the 
stone were perfectly clear, they con- 
tented themselves with polishing the 
natural facets ; but if it contained 
flaws or specks, they covered it with 
numerous small facets haphazardly 
placed. The stone was invariably 
left in almost its original shape, 
and no effort was made to improve the symmetry. 

For a long time little further progress wasmade, 
and even nearly a century after Berquem the only 
regular patterns known to Kentmann, who wrote in 
1 562, were the diamond-point and the diamond-table 
(Figs. 4344). The former consisted of the natural 

octahedron facets ground to regular 

/ \ shape, and was long employed for 

~~x the minute stones which were set 

in conjunction with large coloured 

FIG Table s ^ ones * n rings. The table repre- 

Cut (side view), sented considerably greater labour 

One corner of the regular octahedron 
was ground down until the artificial facet thus pro- 
duced was half the width of the stone, while the 
opposite corner was slightly ground. 

Still another century elapsed before the introduc- 
tion of the rose pattern, which comprised twenty- 
four triangular facets and a flat base (Figs. 4546), 
the stone being nearly hemispherical in shape. This 




92 GEM-STONES 

style is said to have been the invention of Cardinal 
Mazarin, but probably he was the first to have 
diamonds of any considerable size cut in this form. 
At the present day only tiny stones are cut as 
roses. 

A few more years passed away, and at length at 
the close of the seventeenth century diamond came 
by its own when Vincenzio Peruzzi, 
a Venetian, introduced the brilliant 
form of cutting, and revealed for the 
first time its amazing c fire.' Except 
for minor changes this form remains 

,FiG. 4 5.-RoseCut tO this day the standard st y le for 

(top view). the shape of diamond, and the word 

brilliant is commonly employed to 

denote diamond cut in this way. So obviously and 

markedly superior is the style to all others that 

upon its discovery the owners of large roses had 

them re-cut as brilliants despite the loss in weight 

necessitated by the change. 

The brilliant form is derived from the old table 
by increasing the number of facets 
and slightly altering the angles 
pertaining to the natural octa- 
hedron. In a perfect brilliant FlG 46< _ Rose Cut 
(Figs. 47-49) there are altogether '(side view). 
58 facets, 33 above and 25 below 
the girdle, as the edge separating the upper and 
lower portions of the stone is termed, which are 
arranged in the following manner. Eight star- 
facets, triangular in shape, immediately surround 
the large table-facet. Next come four large 
templets or bezels, quadrilateral in form, arranged 
in pairs on opposite sides of the table-facet, the 



FASHIONING OF GEM-STONES 



93 



four quoins or lozenges, similar in shape, coming 
intermediately between them ; in modern practice, 
however, these two sets are identical in shape and 
size, and there are consequently eight facets of the 
same kind instead of two sets of four. The eight 




FlG. 47. Brilliant 
Cut (top view). 




FIG. 48. Brilliant 
Cut (base view). 




cross or skew facets and the eight skill facets, in 
both sets the shape being triangular, form the 
boundary of the girdle ; modern brilliants usually have 
instead sixteen facets of the same shape and size. 
The above 33 facets lie above the girdle and form 
the crown of the stone. Imme- 
diately opposite and parallel to 
the table is the tiny culet. Next 
to the latter come the four large 
pavilion facets with the four quoins 
intermediately between them, both 
sets being five-sided but nearly 
quadrilateral in shape ; these again are usually com- 
bined into eight facets of the same size. Eight cross 
facets and eight skill facets, both sets, like those in 
the crown, being triangular in shape, form the lower 
side of the girdle ; these also are generally united 
into a set of sixteen similar facets. These 2 5 facets 
which lie below the girdle comprise the ' pavilion,' 



FIG. 49. Brilliant 
Cut (side view). 



94 GEM-STONES 

or base of the stone. In a regular stone properly 
cut a templet is nearly parallel to a pavilion, and 
an upper to a lower cross facet. The contour of the 
girdle is usually circular, but occasionally assumes 
less symmetrical shapes, as for instance in drop- 
stones or pendeloques, and the facets are at the 
same time distorted. The number of facets may 
with advantage be increased in the case of large 
stones. An additional set of eight star facets is 
often placed round the culet, the total number then 
being 66. It may be mentioned that the largest 
stone cut from the Cullinan has the exceptional 
number of 74 facets. 

In order to secure the finest optical effect certain 
proportions have been found necessary. The depth 
of the crown must be one-half that of the base, and 
therefore one-third the total depth of the stone, and 
the width of the table must be slightly less than 
half that of the stone. The culet should be quite 
small, not more in width than one-sixth of the 
table ; it is, in fact, not required at all except to 
avoid the danger of the point splintering. The 
girdle should be as thin as is compatible with 
strength sufficient to prevent chipping in the process 
of mounting the stone ; if it were left thick, the 
rough edge would be visible by reflection at the 
lower facets, and would, especially if at all dirty, 
seriously affect the quality of the stone. The shape 
of the stone is largely determined by the sizes of 
the templets in the crown and the pavilions in the 
base as compared with that of the table, or, what 
comes to the same thing, by the inclinations at 
which they are cut to that facet. If the table 
had actually half the width of the stone, the 



FASHIONING OF GEM-STONES 95 

angle l between it and a templet would be exactly 
half a right angle or 45; it is, however, made 
somewhat smaller, namely, about 40. A pavilion, 
being parallel to a templet, makes a similar angle 
with the culet. The cross facets are more 
steeply inclined, and make an angle of about 45 
with the table or the culet, as the case may be. 
The star facets, on the other hand, slant per- 
ceptibly less, and make an angle of only about 26 
with the table. A latitude of some 4 or 5 is 
possible without seriously affecting the ' fire ' of the 
stone. 

The object of the disposition of the facets on a 
brilliant is to assure that all the light that enters 
the stone, principally by way of the table, is wholly 
reflected from the base and emerges through the 
crown, preferably by way of the inclined facets. A 
brilliant-cut diamond, if viewed with the table between 
the observer and the light, appears quite dark except 
for the small amount of light escaping through the 
culet. Light should therefore fall on the lower facets 
at angles greater than the critical angle of total- 
reflection, which for diamond is 24 26'. The 
pavilions should be inclined properly at double this 
angle, or 48 52', to the culet; but a ray that 
emerges at a pavilion in the actual arrangement 
entered the table at nearly grazing incidence, and 
the amount of light entering this facet at such acute 
perspective is negligible. On the other hand, 
after reflection at the base light must, in order to 
emerge, fall on the crown at less than the critical angle 

1 In accordance with the usual custom the angle between the facets 
is taken as that between their normals, or the supplement of the salient 
angle. 



9 6 



GEM-STONES 



of total-reflection. In Fig. 50 are shown diagram 
matically the paths of rays that entered the table 
in divers ways. The ray emerging again at the 
table suffers little or no dispersion and is almost 
white, but those coming out through the inclined 
facets are split up into the rainbow effect, known as 
'fire,' for which diamond is so famous. It is in 
order that so much of the light entering by the 




FlG. 50. Course of the Rays of Light passing through a Brilliant. 

table may emerge through the inclined facets of 
the crown that the pavilions are inclined at not 
much more than 40 to the culet. It might be 
suggested that instead of being faceted the stone 
should be conically shaped, truncated above and 
nearly complete below. The result would no doubt 
be steadier, but, on the other hand, far less pleasing 
It is the ever-changing nuance that chiefly attracts 
the eye ; now a brilliant flash of purest white, anon 



FASHIONING OP GEM-STONES 9? 

a gleam of cerulean blue, waxing to richest orange 
and dying in a crimson glow, all intermingled with 
the manifold glitter from the surface of the stone. 
Absolute cleanliness is essential if the full beauty of 
any stone is to be realized, but this is particularly 
true of diamond. If the back of the stone be 
clogged with grease and dirt, as so often happens 
in claw-set rings, light is no longer wholly reflected 
from the base ; much of it escapes, and the amount 
of ' fire ' is seriously diminished. 

Needless to state, lapidaries make no careful 
angular measurements when cutting stones, but judge 
of the position of the facets entirely by eye. It 
sometimes therefore happens that the permissible 
limits are overstepped, in which event the stone is 
dead and may resist all efforts to vivify it short of 
the heroic course of re-cutting it, too expensive a 
treatment in the case of small stones. 

The factors that govern the properties of a 
brilliant-cut stone are large colour-dispersion, high 
refraction, and freedom from any trace of intrinsic 
colour. The only gem-stone that can vie with 
diamond in these respects is zircon. Although it is 
rare to find a zircon naturally without colour, yet 
many kinds are easily deprived of their tint by the 
application of heat. A brilliant-cut zircon is, indeed, 
far from readily distinguished by eye from diamond, 
and has probably often passed as one, but it may 
easily be identified by its large double refraction 
(cf. p. 41) and inferior hardness. The remaining 
colourless stones, such as white sapphire, topaz, and 
quartz (rock-crystal), have insufficient refractivity to 
give total-reflection at the base, and, moreover, they 
are comparatively deficient in ' fire.' 
7 



98 GEM-STONES 

A popular style of cutting which is much in 
vogue for coloured stones is the step- or trap-cut, 
consisting of a table and a series of facets with 
parallel horizontal edges (Figs. 5152) above and 
below the girdle ; in recent jewellery, however, the top 
of the stone is often brilliant-cut The contour may 
be oblong, square, lozenge, or heart-shaped, or have 
less regular forms. The table is 
sometimes slightly rounded. Since 
the object of this style is primarily 
to display the intrinsic colour of 




view). brilliant play of light from the 

interior, no attempt is made to 
secure total-reflection at the lower facets. The 
stone therefore varies in depth according to its 
tint ; if dark, it is cut shallow, lest light be wholly 
absorbed within, and the stone appear practically 
opaque, but if light, it is cut deep, in order to 
secure fullness of tint. Much precision in shape 
and disposition of the facets is 
not demanded, and the stones are 
usually cut in such a way that, 
provided the desired effect is ob- FlG ;, S2.-Ste P - or 

r . Trap -Cut (side 

tamed, the weight is kept as great view). 
as possible; we may recall that 
stones are sold by weight In considering what 
will be the optical effect of any particular shape, 
regard must be had to the effective colour of the 
transmitted light. For instance, although sapphire 
and ruby belong to the same species and have 
the same refractive indices, yet, since the former 
transmits mainly blue and the latter red light, they 
have for practical purposes appreciably different 



FASHIONING OF G&M-STONES 99 

indices, and lapidaries find it therefore possible to 
cut the base of ruby thicker than that of sapphire, 
and thus keep the weight greater. It is instructive 
too what can be done with the most unpromising 
material by the exercise of a little ingenuity. 
Thus Ceylon sapphires are often so irregularly 
coloured that considerable skill is called for in 
cutting them. A stone may, for instance, be 
almost colourless except for a single spot of blue ; 
yet, if the stone be cut steeply and the spot be 
brought to the base, the effect will be precisely the 
same as if the stone were uniformly coloured, 
because all the light emerging from the stone has 
passed through the spot at the base and therefore 
been tinted blue. 

The mechanism employed in the fashioning of 
gem-stones is simple in character, and comprises 
merely metal plates or wheels for slitting, and discs 
or laps for grinding and polishing the stones, the 
former being set vertically and rotated about 
horizontal spindles, and the latter set horizontally 
and rotated about vertical spindles. Mechanical 
power is occasionally used for driving both kinds 
of apparatus, but generally, especially in slitting 
and in delicate work, hand-power is preferred. In 
the East native lapidaries make use of vertical wheels 
(Plate XIII) also for grinding and polishing stones, 
which explains why native-cut stones never have 
truly plane facets ; it will be noticed from the 
picture that a long bow is used to drive the 
spindle. 

Owing to the unique hardness of diamond it can 
be fashioned only by the aid of its own powder. 
The process differs therefore materially from the 



ioo GEM-STONES 

cutting of the remaining gem-stones, and will be 
described separately. Indeed, so different are the 
two classes of work that firms seldom habitually 
undertake both. 

The discovery of the excellent cleavage of 
diamond enormously reduced the labour of cutting 
large stones. A stone containing a bad flaw may 
be split to convenient shape in as many minutes 
as the days or even weeks required to grind it down. 
The improvement in the appliances and the provision 
of ample mechanical power has further accelerated 
the process and reduced the cost. Two years were 
occupied in cutting the diamond known as the Pitt 
or Regent, whereas in only six months the colossal 
Cullinan was shaped into two large and over a 
hundred smaller stones with far less loss of material. 

Although the brilliant form was derived from the 
regular octahedron, it by no means follows that, 
because diamond can be cleaved to the latter form, 
such is the initial step in fashioning the rough mass. 
The aim of the lapidary is to cut the largest possible 
stone from the given piece of rough, and the finished 
brilliant usually bears no relation whatever to the 
natural octahedron. The cleavage is utilized only 
to free the rough of an awkward and useless excres- 
cence, or of flaws. Although the octahedron is one 
of the common forms in which diamond is found, it 
is rarely regular, and oftener than not one of the 
larger faces is made the table. 

The old method, which is still in use, for roughly 
fashioning diamonds is that known as bruting, from 
the French word, bmtage, for the process, or as 
shaping. Two stones of about the same size are 
selected, and are firmly attached by means of a hard 



FASHIONING OF GEM-STONES 101 

cement to the ends of two holders, which are held one 
in each hand, and rubbed hard, one against the other, 
until surfaces of the requisite size are developed on 
each stone. During the process the stones are held 
over a small box, which catches the precious powder. 
A fine sieve at the bottom of the box allows the 
powder to fall through into a tray underneath, but 
holds back anything larger. By means of two vertical 
pins placed one on each side of the box the holders 
are retained more easily in the desired position, and 
the work is thrown mainly on the thumbs. This 
work continued day after day has a very disfiguring 
effect upon the hands despite the thick gloves that 
are worn to protect them ; the skin of the thumbs 
grows hard and horny, and the first and second 
fingers become swollen and distorted. When the 
surfaces have thus been formed, the stone is handed 
to the polisher, who works them into the correct 
shape and afterwards polishes them, the stone 
passing backwards and forwards several times 
between the cutter and the polisher. The table, 
four templets, culet and four pavilions are first 
formed and polished, so that the table has a square 
shape. Next the quoins are developed and polished, 
and finally the small facets are polished on, not 
being shaped first. In modern practice the process 
of bruting has been modified in some cases by the 
introduction of machinery, and the facets are ground 
on, with considerable improvement in the regularity 
of their size and disposition, and reduction in the 
amount of polishing required. Moreover, to obviate 
the loss of material resulting from continued grinding, 
large stones are first sliced by means of rapidly-re- 
volving copper wheels charged with diamond powder. 



102 GEM-STONES 

The laps used for polishing diamonds are made 
of a particular kind of soft iron, which is found to 
surpass any other metal in retaining the diamond 
powder. They are rotated at a high rate of speed, 
which is about 2000 to 2500 revolutions a minute, 
and the heat developed by the friction at this speed 
is too great for a cement to be used ; a solder or 
fusible alloy, composed of one part tin to three 
parts lead, therefore takes its place. The solder 
is held in a hollow cup of brass which is from 
its shape called a ' dop,' an old Dutch word meaning 
shell. Its external diameter is ordinarily about i| 
in. (4 cm.), but larger dops are, of course, used 
for large stones. A stout copper stalk is attached 
to the bottom of the dop ; it is visible in the view 
of the dop shown at e on Plate VI, and two slabs of 
solder are seen lying in front of the dop. The dop 
containing the solder is placed in the midst of a 
non-luminous flame and heated until the solder 
softens, when it is removed by means of the small 
tongs, c, and placed upright on a stand such as 
that shown at a. The long tongs, d, are used for 
shaping the solder into a cone at the apex of which 
the diamond is placed. The solder is worked well 
over the stone so that only the part to undergo 
polishing is exposed. A diamond in position is 
shown at/. The top of the stand is saucer-shaped 
to catch the stone should it accidentally fall off the 
dop, and to prevent pieces of solder falling on the 
hand. While still hot, the dop with the diamond in 
position on the solder is plunged into cold water in 
order to cool it. The fact that the stone withstands 
this drastic treatment is eloquent testimony to its 
good thermal conductivity ; other gem-stones would 




POLISHING DIAMONDS 



FASHIONING OF GEM-STONES 103 

promptly split into fragments. It may be remarked 
that so high is the temperature at which diamond 
burns that it may be placed in the gas flame without 
any fear of untoward results. The dop is now ready 
for attachment to an arm such as that shown at b ; 
the stalk of the dop is placed in a groove running 
across the split end of the arm, and is gripped tight 
by means of a screw worked by the nut which is 
visible in the picture. 

Four such arms, each with a dop, are used with 
the polishing lap (Plate VII), and each stands on 
two square legs on the bench. Pins, /, in pairs 
are fixed to the bench to prevent the arms being 
carried round by the friction ; one near the lap holds 
the arm not far from the dop, and the other engages 
in a strong metal tongue, which is best seen at the 
end of the arm b on Plate VI. Though the arm, 
which is made of iron, is heavy, yet for polishing 
purposes it is insufficient, and additional lead weights 
are laid on the top of it, as in the case of the arm at 
the back on Plate VII. The copper stalk is strong, 
yet flexible, and can be bent to suit the position of 
the facet to be polished; on Plate VII the dops a 
and b are upright, but the other two are inclined. 
In addition to the powder resulting from bruting, 
boart, i.e. diamonds useless for cutting, are crushed 
up to supply polishing material, and a little olive oil 
is used as a lubricant. Owing to the friction so 
much heat is developed that even the solder would 
soften after a time, and therefore, as a precaution, 
the dop is from time to time cooled by immersion 
in water. The stone has constantly to be re-set, 
about six being the maximum even of the tiny 
facets near the girdle that can be dealt with by 



1 04 GEM-STONES 

varying the inclination of the dop. As the work 
approaches completion the stone is frequently in- 
spected, lest the polishing be carried too far for the 
development of the proper amount of ' fire.' When 
finished, the stones are boiled in sulphuric acid to 
remove all traces of oil and dirt. 

The whole operation is evidently rough and ready 
in the extreme ; but such amazing skill do the 
lapidaries acquire, that even the most careful in- 
spection by eye alone would scarce detect any want 
of proper symmetry in a well-cut stone. 

The fashioning of coloured stones, as all the 
gem-stones apart from diamond are termed in the 
jewellery trade, is on account of their inferior 
hardness a far less tedious operation. They are 
easily slit, for which purpose a vertical wheel 
(Plate VIII) made of soft iron is used ; it is charged 
with diamond dust and lubricated with oil, generally, 
paraffin. When slit to the desired size, the stone is 
attached to a conveniently shaped holder by means 
of a cement, the consistency of which varies with 
the hardness of the stone. It is set in the cement 
in such a way that the plane desired for the table 
facet is at right angles to the length of the holder, 
and the whole of the upper part or crown is finished 
before the stone, is removed from the cement. The 
lower half or base is treated in a similar manner. 
Thus in the process of grinding and polishing the 
stone is only once re-set ; as was stated above, 
diamond demands very different treatment. Again, 
all coloured stones are ground down without any 
intermediate operation corresponding to bruting. 
The holder is merely held in the hand, but to 
maintain its position more exactly its other end, 






PLATE VIII 





ING COLOURED STi'NKS 




VCETING MACHI 



FASHIONING OF GEM-STONES 105 

which is pointed, is inserted in one of the holes that 
are pierced at intervals in a vertical spindle placed 
at a convenient distance from the lap (Plate VIII), 
which one depending upon the inclination of the 
facet to be formed. For hard stones, such as ruby 
and sapphire, diamond powder is generally used as 
the abrasive agent, while for the softer stones emery, 
the impure corundum, is selected ; in recent years 
the artificially prepared carborundum, silicide of 
carbon corresponding to the formula CSi, which is 
harder than corundum, has come into vogue for 
grinding purposes, but it is unfortunately useless 
for slitting, because it refuses to cling to the wheel. 
To efface the scratches left by the abrasive agent 
and to impart a brilliant polish to the facets, 
material of less hardness, such as putty-powder, 
pumice, or rouge, is employed ; in all cases the 
lubricant is water. The grinding laps are made 
of copper, gun-metal, or lead ; and pewter or wooden 
laps, the latter sometimes faced with cloth or 
leather, are used for polishing. As a general 
rule, the harder the stone the greater the speed of 
the lap. 

As in the case of diamond, the lapidary judges of 
the position of the facet entirely by eye and touch, 
but a skilled workman can develop a facet very 
close to the theoretical position. During recent years 
various devices have been invented to enable him to 
do his work with greater facility. A machine of 
this kind is illustrated on Plate IX. The stone is 
attached by means of cement to the blunt end, d, of 
the holder, b, which is of the customary kind, while 
the other end is inserted in a hole in a wooden 
piece, a } which is adjustable in height by means of 



io6 GEM-STONES 

the screw above it. The azimuthal positions of the 
facets are arranged by means of the octagonal collar, 
c, the sides of which are held successively in turn 
against the guide, e. The stand itself is clamped 
to the bench. The machine is, however, little 
used except for cheap stones, because it is too 
accurate and leads to waste of material. Stones are 
sold by weight, and so long as the eye is satisfied, 
no attempt is made to attain to absolute symmetry 
of shape. 

The pictures on Plates X-XIII illustrate lapidaries' 
workshops in various parts of the world. The first 
two show an office and a workshop situated in 
Hatton Garden, London ; in the former certain of 
the staff are selecting from the parcels stones suit- 
able for cutting. The third depicts a more primitive 
establishment at Ekaterinburg in the Urals. The 
fourth shows a typical French family pere, mere, et 
fits in the Jura district, all busily engaged ; on 
the table will be noticed a faceting machine of the 
kind described above. In the fifth picture a native 
lapidary in Calcutta is seen at work with the driving 
bow in his right, and the stone in his left, hand. 

A curious difference exists in the systems of 
charging for cutting diamonds and coloured stones. 
The cost of cutting the latter is reckoned by the 
weight of the finished stone, the rate varying from 
is. to 8s. a carat according to the character of the 
stone and the difficulty of the work ; while in the 
case of diamonds, on the other hand, the weight of 
the rough material determines the cost, the rate 
being about los. to 403. a carat according to the 
size, which on the average is equivalent to about 
303. to 1 2 os. a carat calculated on the weight 



FASHIONING OF GEM-STONES 107 

of the finished stone. The reason of the distinction 
is obviously because the proper proportions in 
a brilliant-cut diamond must be maintained, 
whatever be the loss in weight involved ; in 
coloured stones the shape is not of such primary 
importance. 

When finished, the stone finds its way with 
others akin to it to the manufacturing jeweller's 
establishment, where it is handed to the setter, who 
mounts it in a ring, necklace, brooch, or whatever 
article of jewellery it is intended for. The metal 
used in the groundwork of the setting is generally 
gold, but platinum is also employed where an 
unobtrusive and untarnishable metal is demanded, 
and silver finds a place in cheaper jewellery, although 
it is seriously handicapped by its susceptibility to 
the blackening influence of the sulphurous fumes 
present in the smoke-laden atmosphere of towns. 
The stone may be either embedded in the metal 
or held by claws. The former is by far the 
safer, but the latter the more elegant, and it has the 
advantage of exposing the stone d jour, to use the 
French jewellers' expression, so that its genuineness 
is more evidently testified. It is very important that 
the claw setting be periodically examined, lest the 
owner one day experience the mortification of finding 
that a valuable stone has dropped out ; gold, owing 
to its softness, wears away in course of time. 

Up to quite recent years modern jewellery was 
justly open to the criticism that it was lacking in 
variety, that little attempt was made to secure 
harmonious association in either the colour or the 
lustre of the gem-stones, and that the glitter of the 
gold mount was frequently far too obtrusive. Gold 



1 08 GEM-STONES 

consorts admirably with the rich glow of ruby, but is 
quite unsuited to the gleaming fire of a brilliant. 
Where the metal is present merely for the mechanical 
purpose of holding the stones in position, it should 
be made as little noticeable as possible. The artistic 
treatment of jewellery is, however, receiving now 
adequate attention in the best Paris and London 
houses. Some recent designs are illustrated on 
Plates IV and V. 



PLA TE XIII 




CHAPTER XIII 
NOMENCLATURE OF PRECIOUS STONES 

THE names in popular use for the principal 
gem-stones may be traced back to very early 
times, and, since they were applied long before the 
determinative study of minerals had become a 
science, their significance has varied at different 
dates, and is even now far from precise. No 
ambiguity or confusion could arise if jewellers 
made use of the scientific names for the species, 
but most of them are unknown or at least 
unfamiliar to those unversed in mineralogy, and to 
banish old-established names is undesirable, even if 
the task were not hopeless. The name selected for 
a gem-stone may have a very important bearing on 
its fortunes. When the love-sick Juliet queried 
' What's in a name ? ' her mind was wandering far 
from jewels ; for them a name is everything. The 
beautiful red stones that accompany the diamond in 
South Africa were almost a drug in the market 
under their proper title garnet, but command a 
ready sale under the misnomer ' Cape-ruby.' To 
many minds there is a subtle satisfaction in the 
possession of a stone which is assumed to be a 
sort of ruby that would be destroyed by the know- 
ledge that the stone really belonged to the Cinderella 
species of gem-stones the despised garnet. For 



no GEM-STONES 

similar reasons it was deemed advisable to offer the 
lustrous green garnet found some thirty and odd 
years ago in the Ural Mountains as ' olivine/ not a 
happy choice since their colour is grass- rather than 
olive-green, apart from the fact that the term is in 
general use in science for the species known in 
jewellery as peridot. 

The names employed in jewellery are largely 
based upon the colour, the least reliable from a 
determinative point of view of all the physical 
characters of gem-stones. Qualifying terms are 
employed to distinguish stones of obviously different 
hardness. ' Oriental ' distinguishes varieties of 
corundum, but does not imply that they necessarily 
came from the East ; the finest gem-stones originally 
reached Europe by that road, and the hardest 
coloured stones consequently received that term of 
distinction. 

Nearly all red stones are grouped under the 
name ruby, which is derived from a Latin word, 
ruber, meaning red, or under other names adapted 
from it, such as rubellite, rubicelle. It is properly 
applied to red corundum ; ' balas ' ruby is spinel, 
which is associated with the true ruby at the Burma 
mines and is similar in appearance to it when cut, 
and ' Cape ' ruby, is, as has been stated above, a 
garnet from South Africa. Rubellite is the lovely 
rose-pink tourmaline, fine examples of which have 
recently been discovered in California, and rubicelle 
is a less pronouncedly red spinel. Sapphire is by 
far the oldest and one of the most interesting of the 
words used in the language of jewels. It occurs in 
Hebrew and Persian, ancient tongues, and means 
blue. It was apparently employed for lapis lazuli 



NOMENCLATURE OF PRECIOUS STONES 1 1 1 

or similar substance, but was transferred to the blue 
corundum upon the discovery of this splendid stone. 
Oblivious of the real meaning of the word, jewellers 
apply it in a quasi-generic sense to all the varieties 
of corundum with the exception of the red ruby, and 
give vent to such incongruous expressions as ' white 
sapphire,' ' yellow sapphire ' ; it is true such stones 
often contain traces of blue colour, but that is not 
the reason of the terms. ' Brazilian ' sapphire is 
blue tourmaline, a somewhat rare tint for this species. 
The curious history of the word topaz will be found 
below in the chapter dealing with the species of that 
name. It has always denoted a yellow stone, and 
at the present day is applied by jewellers indis- 
criminately to the true topaz and citrine, the yellow 
quartz, the former, however, being sometimes dis- 
tinguished by the prefix ' Brazilian.' ' Oriental ' 
topaz is corundum, and 'occidental' topaz is a 
term occasionally employed for the yellow quartz. 
Emerald, which means green, was first used for 
chrysocolla, an opaque greenish stone (p. 288), but 
was afterwards applied to the priceless green variety 
of beryl, for which it is still retained. ' Oriental ' 
emerald is corundum, c Brazilian ' emerald in the 
eighteenth century was a common term for the 
green tourmaline recently introduced to Europe, and 
' Uralian ' emerald has been tentatively suggested 
for the green garnet more usually known as 
'olivine.' Amethyst is properly the violet quartz, 
but with the prefix ' oriental ' it is also applied to 
violet corundum, though some jewellers, use it for 
the brilliant quartz, with purple and white sectors, 
from Siberia. Almandine, which is derived from the 
name of an Eastern mart for precious stones, has 



ii2 GEM-STONES 

come to signify a stone of columbine-red hue, 
principally garnet, but with suitable qualification 
corundum and spinel also. 

The nomenclature of jewellery tends to suggest 
relations between the gem-stones for which there is 
no real foundation, and to obscure the essential 
identity, except from the point of view of colour, 
of sapphire and ruby, emerald and aquamarine, 
cairngorm and amethyst. 



CHAPTER XIV 
MANUFACTURED STONES r 

THE initial step in the examination of a 
crystallized substance is to determine its 
physical characters and to resolve it by chemical 
analysis into its component elements ; the final, and 
by far the hardest, step is to build it up or synthetic- 
ally prepare it from its constituents. Unknown to 
the world at large, work of the latter kind has long 
been going on within the walls of laboratories, and 
as the advance in knowledge placed in the hands 
of experimenters weapons more and more compar- 
able with those wielded by nature, their efforts have 
been increasingly successful. So stupendous, how- 
ever, are the powers of nature that the possibility 
of reproducing, by human agency, the treasured 
stones which are extracted from the earth in various 
parts of the globe at the cost of infinite toil and 
labour has always been derided by those ignorant 
of what had already been accomplished. Great, 
therefore, was the consternation and the turmoil 
when concrete evidence that could not be gainsaid 
showed that man's restless efforts to bridle nature 
to his will were not in vain, and congresses of 
all the high-priests of jewellery were hastily con- 
vened to ban such unrighteous products, with what 
ultimate success remains to be seen. 
8 " 3 



114 GEM-STONES 

Crystallization may be caused in four different 
ways, of which the second alone has as yet yielded 
stones large enough to be cut 

1. By the separation of the substance from a 
saturated solution. In nature the solvent may not 
be merely hot water, or water charged with an acid, 
but molten rock, and the temperature and the 
pressure may be excessively high. 

2. By the solidification of the liquefied substance 
upon cooling. Ice is a familiar example of this 
type. 

3. By the sublimation of the vapour of the sub- 
stance, which means the direct passage from the 
vapour to the solid state without traversing the 
usually intervening liquid state. It is usually the 
most difficult of attainment of the four methods ; 
the most familiar instance is snow. 

4. By the precipitation of the substance from a 
solution when set free by chemical action. 

Other things being equal, the simpler the com- 
position the greater is the ease with which a sub- 
stance may be expected to be formed ; for, instead 
of one complex substance, two or more different 
substances may evolve, unless the conditions are 
nicely arranged. Attempts, for instance, to produce 
beryl might result instead in a mixture of chryso- 
beryl, phenakite, and quartz. 

By far the simplest in composition of all the 
precious stones is diamond, which is pure crystallized 
carbon ; but its manufacture is attended by well- 
nigh insuperable difficulties. If carbon be heated 
in air, it burns at a temperature well below its 
melting point ; moreover, unless an enormously 
high pressure is simultaneously applied, the product 



MANUFACTURED STONES 1 1 5 

is the other form of crystallized carbon, namely, the 
comparatively worthless graphite. Moissan's in- 
teresting course of experiments were in some degree 
successful, but the tiny diamonds were worthless 
as jewels, and the expense involved in their manu- 
facture was out of all proportion to any possible 
commercial value they might have. 

Next to diamond the simplest substances among 
precious stones are quartz (crystallized silica) and 
corundum (crystallized alumina). The crystallization 
of silica has been effected in several ways, but the 
value in jewellery of quartz, even of the violet 
variety, amethyst, is not such as to warrant its 
manufacture on a commercial scale. Corundum, 
on the other hand, is held in high esteem ; rubies 
and sapphires, of good colour and free from flaws, 
have always commanded good prices. The question 
of their production by artificial means has therefore 
more than academic interest. 

Ever since the year 1837, when Gaudin produced 
a few tiny flakes, French experimenters have steadily 
prosecuted their researches in the crystallization of 
corundum. Frdmy and Feil, in 1877, were the 
first to meet with much success. A portion of one 
of their crucibles lined with glistening ruby flakes 
is exhibited in the British Museum (Natural 
History). 

In 1885 the jewellery market was completely 
taken by surprise by the appearance of red stones, 
emanating, so it is alleged, from Geneva ; having 
the physical characters of genuine rubies, they were 
accepted as, and commanded the prices of, the 
natural stones. It was eventually discovered that 
they had resulted from the fusion of a number of 



n6 



GEM-STONES 



fragments of natural rubies in the oxy-hydrogen 
flame. The original colour was driven off at that 
high temperature, but was revived by the previous 
addition of a little bichromate of potassium. Owing 
to the inequalities of growth, the cracks due to 
rapid cooling, the inclusion of 
air-bubbles, often so numerous 
as to cause a cloudy appear- 
ance, and, above all, the un- 
natural colour, these recon- 
structed stones, as they are 
termed, were far from satisfac- 
tory, but yet they marked such 
an advance on anything that 
had been accomplished before 
that for some time no suspicion 
was aroused as to their being 
other than natural stones. 

A notable advance in the 
synthesis of corundum, par- 
ticularly of ruby, was made in 
1 904, when Verneuil, who had 
served his apprenticeship to 
science under the guidance of 
Fremy, invented his ingenious 
inverted form of blowpipe 
( Fi g- 53)> which enabled him 
to overcome the difficulties that had baffled earlier 
investigators, and to manufacture rubies vying 
in appearance after cutting with the best of 
nature's productions. The blowpipe consisted of 
two tubes, of which the upper, E, wide above, was 
constricted below, and passing down the centre 
of the lower, F, terminated just above the orifice 




FIG. 53. Verneuil's In- 
verted Blowpipe. 



MANUFACTURED STONES 117 

of the latter in a fine nozzle. Oxygen was admitted 
at C through the plate covering the upper end of 
the tube, E. A rod, which passed through a rubber 
collar in the same plate, supported inside the tube, 
E, a vessel, D, and at the upper end terminated in 
a small plate, on which was fixed a disc, B. The 
hammer, A, when lifted by the action of an electro- 
magnet and released, fell by gravity and struck the 
disc. The latter could be turned about a horizontal 
axis placed eccentrically, so that the height through 
which the hammer fell and the consequent force of 
the blow could be regulated. The rubber collar, 
which was perfectly gas-tight, held the rod securely, 
but allowed the shocks to be transmitted to the 
vessel, D, an arrangement of guides maintaining 
the slight motion of the vessel strictly vertical. 
This vessel, which carried the alumina powder used 
in the manufacture of the stone, had as its base a 
cylindrical sieve of fine mesh. The succession of 
rapid taps of the hammer caused a regular feed of 
powder down the tube, the amount being regulated 
by varying the height through which the hammer 
fell. Hydrogen or coal-gas was admitted at G 
into the outer tube, F, and in the usual way met 
the oxygen just above the orifice, L. To exclude 
irregular draughts, the flame was surrounded by a 
screen, M, which was provided with a mica window, 
and a water-jacket, K, protected the upper part of 
the apparatus from excessive heating. 

The alumina was precipitated from a solution of 
pure ammonia - alum, (NH 4 ) 2 SO 4 .A1 2 (SO 4 ) 3 .24H 2 O, 
in distilled water by the addition of pure ammonia, 
sufficient chrome-alum also being dissolved with 
the ammonia-L^um to furnish about 2\ per cent. 



1 1 8 GEM-STONES 

of chromic oxide in the resulting stone. The 
powder, carefully prepared and purified, was placed, 
as has been stated above, in the vessel, D, and on 
reaching the flame at the orifice it melted, and fell 
as a liquid drop, N, upon the pedestal, P, which 
was formed of previously fused alumina. This 
pedestal was attached by a platinum sleeve to an 
iron rod, Q, which was provided with the necessary 
screw adjustments, R and S, for centring and 
lowering it as the drop grew in size. Great care 
was exercised to free the powder from any trace of 
potassium, which, if present, imparted a brownish 
tinge to the stone. The pressure of the oxygen, 
low initially both to prevent the 
pedestal from melting, and to keep 
the area of the drop in contact with 
the pedestal as small as possible, 
FIG. 54. 'Boule,' because otherwise flaws tended to 
D Car S aped start on cooling, was gradually in- 
creased until the flame reached the 
critical temperature which kept the top of the drop 
melted, but not boiling. The supply of powder was at 
the same time carefully proportioned to the pressure. 
The pedestal, P, was from time to time lowered, and 
the drop grew in the shape of a pear (Fig. 54), the apex 
of which was downwards and adhered to the pedestal 
by a narrow stalk. As soon as the drop reached 
the maximum size possible with the size of the 
flame, the gases were sharply and simultaneously 
cut off. After ten minutes or so the drop was 
lowered from the chamber, M, by the screw, S, and 
when quite cold was removed from the pedestal. 

Very few changes have been made in the method 
when adapted to commercial use. Coal-gas has, 




BI.OWI'II'E USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES 



MANUFACTURED STONES 119 

however, entirely replaced the costly hydrogen, and 
the hammer is operated by a cam instead of an 
electromagnet, while, as may be seen from the view 
of a gem-stone factory (Plate XIV), a number of blow- 
pipes are placed in line so that their cams are 
worked by the same shaft, a. The fire-clay screen, 
b, surrounding the flame is for convenience of re- 
moval divided into halves longitudinally, and a 
small hole is left in front for viewing the stone 
during growth, a red glass screen, c, being provided 
in front to protect the eyes from the intense glare. 
Half the fire-clay screen of the blowpipe in the 
centre of the Plate has been removed to show the 
arrangement of the interior. The centring and 
the raising and lowering apparatus, d, have been 
modified. The process is so simple that one man 
can attend to a dozen or so of these machines, and 
it takes only one hour to grow a drop large enough 
to be cut into a ten-carat stone. 

The drops, unless the finished stone is required 
to have a similar pear shape, are divided longitudin- 
ally through the central core into halves, which in 
both shape and orientation are admirably suited to 
the purposes of cutting ; as a general rule, the drop 
splits during cooling into the desired direction of 
its own accord. 

Each drop is a single crystalline individual, and 
not, as might have been anticipated, an alumina 
glass or an irregular aggregation of crystalline 
fragments, and, if the drop has cooled properly, 
the crystallographic axis is parallel to the core of 
the pear. The cut stone will therefore have not 
only the density and hardness, but also all the 
optical characters refractivity, double refraction, 




120 GEM-STONES 

dichroism, etc. pertaining to the natural species, 
and will obey precisely the same tests with the re- 
fractometer and the dichroscope. Were it not for 
certain imperfections it would be impossible to 
distinguish between the stones formed in Nature's 
vast workshop and those produced 
within the confines of a laboratory. 
The artificial stones, however, are 
rarely, if ever, free from minute 
air-bubbles (Fig. 55), which can 
FIG. 55. Bubbles easily be seen with an ordinary 
and Curved Strise lens Their sp herical shape differ- 
in Manufactured . , .. , , 
Ruby. entiates them from the plane- 
sided cavities not infrequently 
visible in a natural stone (Fig. 56). Moreover, 
the colouring matter varies slightly, but imper- 
ceptibly, in successive shells, and consequently in 
the finished stone a careful eye can discern the 
curved striations (Fig. 55) corresponding in shape 
to the original shell. In a natural 
stone, on the other hand, although 
zones of different colours or varying 
shades are not uncommon, the 
resulting striations are straight 
(Fig. 56), corresponding to the ^ 
plane faces of the original crystal in Natura i Ruby. 
form. By sacrificing material it 
might be possible to cut a small stone free from 
bubbles, but the curved striations would always be 
present to betray its origin. 

The success that attended the manufacture of 
ruby encouraged efforts to impart other tints to 
crystallized alumina. By reducing the percentage 
amount of chromic oxide, pink stones were turned 




MANUFACTURED STONES 121 

out, in colour not unlike those Brazilian topazes, the 
original hue of which has been altered by the appli- 
cation of heat. These artificial stones have there- 
fore been called ' scientific topaz ' ; of course, quite 
wrongly, since topaz, which is properly a fluo-silicate 
of aluminium, is quite a different substance. 

Early attempts made to obtain the exquisite blue 
tint of the true sapphire were frustrated by an un- 
expected difficulty. The colouring matter, cobalt 
oxide, was not diffused evenly through the drop, 
but was huddled together in splotches, and it was 
found necessary to add a considerable amount of 
magnesia as a flux before a uniform distribution of 
colour could be secured. It was then discovered 
that, despite the colour, the stones had the physical 
characters, not of sapphire, but of the species closely 
allied to it, namely, spinel, aluminate of magnesium. 
By an unsurpassable effort of nomenclature these 
blue stones were given the extraordinary name of 
' Hope sapphire,' from fanciful analogy with the 
famous blue diamond which was once the pride of 
the Hope collection. A blue spinel is occasionally 
found in nature, but the actual tint is somewhat 
different. These manufactured stones have the 
disadvantage of turning purple in artificial light. 
By substituting lime for magnesia as a flux, Paris, 
a pupil of Verneuil's, produced blue stones which 
were not affected to the same extent. The difficulty 
was at length overcome at the close of 1909, when 
Verneuil, by employing as tinctorial agents 0*5 per 
cent, of titanium oxide and 1-5 per cent, of magnetic 
iron oxide, succeeded in producing blue corundum ; 
it, however, had not quite the tint of sapphire. 
Stones subsequently manufactured, which were 



122 GEM-STONES 

better in colour, contained about 0-12 per cent 
of titanium oxide, but no iron at all. 

By the addition to the alumina of a little nickel 
oxide and vanadium oxide respectively, yellow and 
yellowish green corundums have been obtained. 
The latter have in artificial light a distinctly reddish 
hue, and have therefore been termed 'scientific 
alexandrite'; of course, quite incorrectly, since the 
true alexandrite is a variety of chrysoberyl, alumin- 
ate of beryllium, a very different substance. 

If no colouring matter at all be added and the 
alum be free from potash, colourless stones or white 
sapphires are formed, which pass under the name 
' scientific brilliant.' It is scarcely necessary to 
remark that they are quite distinct from the true 
brilliant, diamond. 

The high prices commanded by emeralds, and 
the comparative success that attended the recon- 
struction of ruby from fragments of natural stones, 
suggested that equal success might follow from a 
similar process with powdered beryl, chromic oxide 
being used as the colouring agent. The resulting 
stones are, indeed, a fair imitation, being even pro- 
vided with flaws, but they are a beryl glass with 
lower specific gravity and refractivity than the true 
beryl, and are wrongly termed ' scientific emerald.' 
Moreover, recently most of the stones so named on 
the market are merely green paste. 

It is unfortunate that the real success which has 
been achieved in the manufacture of ruby and sap- 
phire should be obscured by the ill-founded claims 
tacitly asserted in other cases. 

At the time the manufactured ruby was a novelty 
it fetched as much as ,6 a carat, but as soon as 



MANUFACTURED STONES 123 

it was discovered that it could easily be differenti- 
ated from the natural stone, a collapse took place, 
and the price fell abruptly to 305., and eventually 
to 5s. and even is. a carat. The sapphires run 
slightly higher, from 2s. to ?s. a carat. The prices 
of the natural stones, which at first had fallen, have 
now risen to almost their former level. The extreme 
disparity at present obtaining between the prices 
of the artificial and the natural ruby renders the 
fraudulent substitution of the one for the other 
a great temptation, and it behoves purchasers to 
beware where and from whom they buy, and to be 
suspicious of apparently remarkable bargains, especi- 
ally at places like Colombo and Singapore where 
tourists abound. It is no secret that some thousands 
of carats of manufactured rubies are shipped annu- 
ally to the East. Caveat emptor. 



CHAPTER XV 
IMITATION STONES 

THE beryl glass mentioned in the previous 
chapter marks the transition stage between 
manufactured stones which in all essential characters 
are identical with those found in nature, and arti- 
ficial stones which resemble the corresponding natural 
stone in outward appearance only. In a sense both 
sorts may be styled artificial, but it would be mis- 
leading to confound them under the same appel- 
lation. 

Common paste, 1 which is met with in drapery 
goods and cheap ornaments in general hat-pins, 
buckles, and so forth is composed of ordinary 
crown-glass or flint-glass, the refractive indices 
being about 1*53 and i'63 respectively. The 
finest quality, which is used for imitations of 
brilliants, is called ' strass.' It is a dense lead flint- 
glass of high refraction and strong colour - dis- 
persion, consisting of 38*2 per cent, of silica, 53^3 
red lead (oxide of lead), and 7'8 potassium carbon- 
ate, with small quantities of soda, alumina, and 
other substances. How admirable these imitations 
may be, a study of the windows of a shop devoted 

1 The word paste is derived from the Italian, pasta, food, being 
suggested by the soft plastic nature of the material used to imitate 
gems. 



IMITATION STONES 125 

to such things will show. Unfortunately the 
addition of lead, which is necessary for imparting 
the requisite refraction and ' fire ' to the strass, 
renders the stones exceedingly soft. All glass 
yields to the file, but strass stones are scratched 
even by ordinary window-glass. If worn in such 
a way that they are rubbed, they speedily lose the 
brilliance of their polish, and, moreover, they are 
susceptible to attack by the sulphurous fumes 
present in the smoky air of towns, and turn after 
a time a dirty brown in hue. When coloured 
stones are to be imitated, small quantities of a 
suitable metallic oxide are fused with the glass ; 
cobalt gives rise to a royal-blue tint, chromium a 
ruby red, and manganese a violet. Common paste 
is not highly refractive enough to give satisfactory 
results when cut as a brilliant, and the bases are 
therefore often coated with quicksilver, or, in the case 
of old jewellery, covered with foil in the setting, in 
order to secure more complete reflection from the 
interior. The fashioning of these imitation stones 
is easy and cheap. Being moulded, they do not 
require cutting, and the polishing of the facets thus 
formed is soon done on account of the softness of 
the stones. 

A test with a file readily differentiates paste 
stones from the natural stones they pretend to be. 
Being necessarily singly refractive, they are, of 
course, lacking in dichroism, and their refractivity 
seldom accords even approximately with that of 
the corresponding natural stone. 

In order to meet the test for hardness the 
doublet was devised. Such a stone is composed 
of two parts the crown consisting of colourless 



126 GEM-STONES 

quartz or other inexpensive real and hard stone, 
and the base being made up of coloured glass. 
When the imitation, say of a sapphire, is intended 
to be more exact, the crown is made of a real 
sapphire, but one deficient in colour, the requisite 
tint being obtained from the paste forming the 
under part of the doublet. In case the base 
should also be tested for hardness the triplet 
has been devised. In this the base is made of a 
real stone also, and the coloured paste is confined 
to the girdle section, where it is hidden by the 
setting. Sapphires and emeralds of indifferent 
colour are sometimes slit across the girdle; the 
interior surfaces are polished, and colouring matter 
is introduced with the cement, generally Canada 
balsam, which is used to re-unite the two portions 
of the stone together. All such imitations may 
be detected by placing the stone in oil, when the 
surfaces separating the portions of the composite 
stone will be visible, or the binding cement may 
be dissolved by immersing the stone, if unmounted, 
in boiling water, or in alcohol or chloroform, when 
the stone will fall to pieces. 

The glass imitations of pearls, which have be- 
come very common in recent years, may, apart from 
their inferior iridescence, be detected by their greater 
hardness, or by the apparent doubling of, say, a spot 
of ink placed on the surface, owing to reflection from 
the inner surface of the glass shell. They are made 
of small hollow spheres formed by blowing. Next 
to the glass comes a lining of parchment size, and 
next the under lining, which is the most important 
part of the imitation, consisting of a preparation of 
fish scales called Essence d' Orient. When the lining 



IMITATION STONES 127 

is dry, the globe is filled with hot wax to impart the 
necessary solidity. In cheap imitations the glass 
balls are not lined at all, but merely heated with 
hydrochloric acid to give an iridescence to the sur- 
face ; sometimes they are coated with wax, which can 
be scraped off with a knife. 



PART II SECTION A 
PRECIOUS STONES 

CHAPTER XVI 
DIAMOND 

DIAMOND has held pride of place as chief 
of precious stones ever since the discovery 
of the form of cutting known as the ' brilliant ' 
revealed to full perfection its amazing qualities ; and 
justly so, since it combines in itself extreme hardness, 
high refraction, large colour-dispersion, and brilliant 
lustre. A rough diamond, especially from river 
gravels, has often a peculiar greasy appearance, and 
is no more attractive to the eye than a piece of 
washing-soda. It is therefore easy to understand 
why the Persians in the thirteenth century placed 
the pearl, ruby, emerald, and even peridot before it, 
and writers in the Middle Ages frequently esteemed 
it below emerald and ruby. The Indian lapidaries, 
who were the first to realize that diamond could 
be ground with its own powder, discovered what 
a wonderful difference the removal of the skin makes 
in the appearance of a stone. They, however, 
made no attempt to shape a stone, but merely 
polished the natural facets, and only added numerous 



DIAMOND 129 

small facets when they wished to conceal flaws or 
other imperfections ; indeed, the famous traveller, 
Tavernier, from whom most of our knowledge of 
early mining in India is obtained, invariably found 
that a stone covered with many facets was badly 
flawed. The full radiant beauty of a diamond 
comes to light only when it is cut in brilliant form. 

Of all precious stones diamond has the simplest 
composition ; it is merely crystallized carbon, another 
form of which is the humble and useful graphite, 
commonly known as ' black-lead.' Surely nature 
has surpassed all her marvellous efforts in producing 
from the same element substances with such 
divergent characters as the hard, brilliant, and 
transparent diamond and the soft, dull, and opaque 
graphite. It is, however, impossible to draw any 
sharp dividing line between the two ; soft diamond 
passes insensibly into hard graphite, and vice versa. 
Boart, or bort, as it is sometimes written, is composed 
of minute crystals of diamond arranged haphazardly ; 
it possesses no cleavage, its hardness is greater than 
that of the crystals, and its colour is greyish to 
blackish. Carbon, carbonado, or black diamond, 
which is composed of still more minute crystals, 
is black and opaque, and is perceptibly harder 
than the crystals. It passes into graphite, which 
varies in hardness, and may have any density 
between 2 - o and 3 - o. Jewellers apply the term 
boart to crystals or fragments which are of no 
service as gems ; such pieces are crushed to powder 
and used for cutting and polishing purposes. 

Diamonds, when absolutely limpid and free from 
flaws, are said to be of the ' first water,' and are 
most prized when devoid of any tinge of colour 
9 



1 30 GEM-STONES 

except perhaps bluish (Plate I, Fig. i). Stones with 
a slight tinge of yellow are termed 'off-coloured,' 
and are far less valuable. Those of a canary-yellow 
colour (Plate I, Fig. 3), however, belong to a different 
category, and have a decided attractiveness. Green- 
ish stones also are common, though it is rare to 
come across one with a really good shade of that 
colour. Brown stones, especially in South Africa, 
are not uncommon. Pink stones are less common, 
and ruby-red and blue stones are rare. Those of 
the last-named colour have usually what is known 




FIGS. 57-59. Diamond Crystals. 

as a ' steely ' shade, i.e. they are tinged with green ; 
stones of a sapphire blue are very seldom met with, 
and such command high prices. 

Diamond crystallizes (Figs. 5759 and Plate I, 
Fig. 2) in octahedra with brilliant, smooth faces, and 
occasionally in cubes with rough pitted faces ; 
sometimes three or six faces take the place of each 
octahedron face, and the stone is almost spherical 
in shape. The surfaces of the crystals are often 
marked with equilateral triangles, which are supposed 
to represent the effects of incipient combustion. 
Twinned crystals, in which the two individuals 
may be connected by a single plane or may be 



DIAMOND 131 

interpenetrating, a star shape often resulting in the 
latter case, are common ; sometimes, if of the 
octahedron type, they are beautifully symmetrical. 
The rounded crystals are frequently covered with 
a peculiar gum-like skin which is somewhat less 
hard than the crystal itself. A large South African 
stone, weighing 27 grams (130 carats) and octahedral 
in shape, which was the gift of John Ruskin, and 
named by him the ' Colenso ' after the first bishop of 
Natal, is exhibited in the British Museum (Natural 
History) ; its appearance is, however, marred by its 
distinctly ' off-coloured ' tint. 

The refraction of diamond is single, but local 
double refraction is common, indicating a state of 
strain which can often be traced to an included 
drop of liquid carbonic acid ; so great is the strain 
that many a fine stone has burst to fragments on 
being removed from the ground in which it has lain. 
The refractive index for the yellow light of a sodium 
flame is 2^4 17 5, and the slight variation from this 
mean value that has been observed, amounting only 
to O'OOOl, testifies to the purity of the composition. 
The colour-dispersion is large, being as much as 
0-044, in which respect it surpasses all colourless 
stones, but is exceeded by sphene and the green 
garnet from the Urals (cf. p. 217). The lustre of 
diamond, when polished, is so characteristic as to 
be termed adamantine, and is due to the combina- 
tion of high refraction and extreme hardness. 
Diamond is translucent to the X (Rontgen) rays ; 
it phosphoresces under the action of radium, and 
of a high-tension electric current when placed in 
a vacuum tube, and sometimes even when exposed 
to strong sunlight. Some diamonds fluoresce in 



132 GEM-STONES 

sunlight, turning milky, and a few even emit light 
when rubbed. Crookes found that a diamond 
buried in radium bromide for a year had acquired 
a lovely blue tint, which was not affected even by 
heating to redness. The specific gravity is like- 
wise constant, being 3*521, with a possible variation 
from that mean value of 0*005 5 but a greater range, 
as might be expected, is found in the impure boart. 
Diamond is by far the hardest substance in nature, 
being marked I o in Mohs's scale of hardness, but 
it varies in itself; stones from Borneo and New 
South Wales are so perceptibly harder than those 
usually in the lapidaries' hands, that they can be 
cut only with their own and not ordinary diamond 
powder, and some difficulty was experienced in 
cutting them when they first came into the market. 
It is interesting to note that the metal tantalum, 
the isolation of which in commercial amount 
constituted one of the triumphs of chemistry of 
recent years, has about the same hardness as 
diamond. Despite its extreme hardness diamond 
readily cleaves under a heavy blow in planes 
parallel to the faces of the regular octahedron, a 
property utilized for shaping the stone previous to 
cutting it. The fallacious, but not unnatural, idea 
was prevalent up to quite modern times that a 
diamond would, even if placed on an anvil, resist 
a blow from a hammer : who knows how many 
fine stones have succumbed to this illusory test? 
The fact that diamond could be split was known to 
Indian lapidaries at the time of Tavernier's visit, 
and it would appear from De Boodt that in the 
sixteenth century the cleavability of diamond was 
not unknown in Europe, but it was not credited 



DIAMOND 133 

at the time and was soon forgotten. Early last 
century Wollaston, a famous chemist and mineral- 
ogist, rediscovered the property, and, so it is said, 
used his knowledge to some profit by purchasing 
large stones, which because of their awkward shape 
or the presence of flaws in the interior were rejected 
by the lapidaries, and selling them back again after 
cleaving them to suitable forms. 

It has already been remarked (p. 79) that the 
interval in hardness between diamond and corundum, 
which comes next to it in Mohs's scale, is enormously 
greater than that between corundum and the softest 
of minerals. Diamond can therefore be cut only 
with the aid of its own powder, and the cutting of 
diamond is therefore differentiated from that of other 
stones, the precious-stone trade being to a large ex- 
tent divided into two distinct groups, namely, dealers 
in diamonds, and dealers in all other gem-stones. 

The name of the species is derived from the 
popular form, adiamentem, of the Latin adamantem, 
itself the alliterative form of the Greek aSa/^a?, 
meaning the unconquerable, in allusion not merely 
to the great hardness but also to the mistaken idea 
already mentioned. Boart probably comes from 
the Old-French bord or bort y bastard. 

At the present day diamonds are usually cut as 
brilliants, though the contour of the girdle may be 
circular, oval, or drop-shaped to suit the particular 
purpose for which the stone is required, or to keep 
the weight as great as possible. Small stones for 
bordering a large coloured stone may also be cut 
as roses or points. A perfect brilliant has 5 8 facets, 
but small stones may have not more than 44, and 
exceptionally large stones may with advantage have 



134 GEM-STONES 

many more ; for instance, on the largest stone cut 
from the Cullinan diamond there are no fewer than 
74 facets. 

The description of the properties of diamond 
would not be complete without a reference to the 
other valuable, if utilitarian, purposes to which it is 
put Without its aid much of modern engineering 
work and mining operations would be impossible 
except at the cost of almost prohibitive expenditure 
of time and money. 

Boring through solid rock has been greatly 
facilitated by the use of the diamond drill. For 
this purpose carbonado or black diamond is more 
serviceable than single crystals, and the price of 
the former has consequently advanced from a 
nominal figure up to 3 to 12 a carat. The actual 
working part of the drill consists of a cast-steel 
ring. The crown of it has a number of small 
depressions at regular intervals into which the 
carbonados are embedded. On revolution of the 
drill an annular ring is cut, leaving a solid core 
which can be drawn to the surface. For cooling 
the drill and for washing away the detritus water 
is pumped through to the working face. The 
duration of the carbonados depends on the nature 
of the rock and the skill of the operator. The most 
troublesome rock is a sandstone or one with sharp 
differences in hardness, because the carbonados are 
liable to be torn out of their setting. An experienced 
operator can tell by the feel of the drill the nature 
of the rock at the working face, and by varying the 
pressure can mitigate the risk of damage to the drill. 

The tenacity of diamond renders it most suitable 
for wire-drawing. The tungsten filaments used in 



DIAMOND 135 

many of the latest forms of incandescent electric 
lamps are prepared in this manner. 

Diamond powder is used for cutting and turning 
the hardened steel employed in modern armaments 
and for other more peaceful purposes. 

Although nearly all the gem-stones scratch glass, 
diamond alone can be satisfactorily employed to 
cut it along a definite edge. Any flake at random 
will not be suitable, because it will tear the glass 
and form a jagged edge. The best results are given 
by the junction of two edges which do not meet in 
too obtuse an angle ; two edges of the rhombic 
dodecahedron meet the requirements admirably. 
The stones used by the glaziers are minute in size, 
being not much larger than .a pin's head, and thirty 
of them on an average go to the carat. They are 
set in copper or brass. Some little skill is needed 
to obtain the best results. 

The value of a diamond has always been determined 
largely by the size of the stone, the old rule being 
that the rate per carat should be multiplied by the 
square of the weight in carats ; thus, if the rate be 
i o, the cost of a two-carat stone is four times this 
sum, or 4.0, of a three-carat stone 90, and so on. 
For a century, from 1750 to 1850, the rate remained 
almost constant at 4. for rough, 6 for rose-cut, 
and 8 for brilliant-cut diamonds. Since the latter 
date, owing to the increase in the supply of gold, 
the growth of the spending power of the world, and 
the gradual falling off in the productiveness of 
the Brazilian fields, the rate steadily increased about 
10 per cent, each year, until in 1865 the rate for 
brilliants was iB. The rise was checked by 
the discovery of the South African mines ; moreover, 



136 GEM-STONES 

since comparatively large stones are plentiful in 
these mines, the rule of the increase in the price of 
a stone by the square of its weight no longer holds. 
The rate for the most perfect stones still remains 
high, because such are not so common in the South 
African mines. The classification 1 adopted by the 
syndicate of London diamond merchants who place 
upon the market the output of the De Beers group 
of mines is as follows : (a) Blue-white, (b} white, 
(c) silvery Cape, (d) fine Cape, (e) Cape, (/") fine by- 
water, (g-) by water, (/t) fine light brown, (z) light 
brown, (/) brown, (/) dark brown. Bywaters or 
byes are stones tinged with yellow. 

The rate per carat for cut stones in the blue- 
white and the by water groups is : 

BLUE-WHITE. BYWATER. 

5-carat stone . . ^40-60 ^20-25 

i . 30-40 10-15 

\ . . 20-25 8-12 

J . 15-18 6-10 

Melee . . . 12-15 5-8 

Melee are stones smaller than a quarter of a carat. 
It will be noticed that the prices depart largely 
from the old rule ; thus taking the rate for a carat 
blue-white stone, the price of a five-carat stone 
should be from 15 0200 a carat, and for a 
quarter-carat stone only 7, los. to 10 a carat. 
There happens to be at the time of writing very 
little demand for five-carat stones. Of course, the 
prices given are subject to constant fluctuation 
depending upon the supply and demand, and the 
whims of fashion. 

1 Cf. below, p. 149. 



CHAPTER XVII 
OCCURRENCE OF DIAMOND 

THE whole of the diamonds known in ancient 
times were obtained from the so-called 
Golconda mines in India. Golconda itself, now a 
deserted fortress near Hyderabad, was merely the 
mart where the diamonds were bought and sold. 
The diamond-bearing district actually spread over a 
wide area on the eastern side of the Deccan, ex- 
tending from the Pinner River in the Madras 
Presidency northwards to the Rivers Son and Khan, 
tributaries of the Ganges, in Bundelkhand. The 
richest mines, where the large historical stones were 
found, are in the south, mostly near the Kistna 
River. The diamonds were discovered in sandstone, 
or conglomerate, or the sands and gravels of river- 
beds. The mines were visited in the middle of the 
seventeenth century by the French traveller and 
jeweller, Tavernier, when travelling on a commission 
for Louis XIV, and he afterwards published a careful 
description of them and of the method of working 
them. The mines seem to have been exhausted in 
the seventeenth century ; at any rate, the prospecting, 
which has been spasmodically carried on during the 
last two centuries, has proved almost abortive. 
With the exception of the Koh-i-nor, all the large 
Indian diamonds were probably discovered not long 



138 GEM-STONES 

before Tavernier's visit. The diamonds known to 
Pliny, and in his time, were quite small, and it is 
doubtful if any stones of considerable size came to 
light before A.D. 1000. 

India enjoyed the monopoly of supplying the 
world's demand for diamonds up to the discovery, 
in 1725, of the precious stone in Brazil. Small 
stones were detected by the miners in the gold 
washings at Tejuco, about eighty miles (129 km.) 
from Rio de Janeiro, in the Serro do Frio district of 
the State of Minas Geraes. The discovery naturally 
caused great excitement. So many diamonds were 
found that in 1727 something like a slump took 
place in their value. In order to keep up prices, 
the Dutch merchants, who mainly controlled the 
Indian output, asserted that the diamonds had not 
been found in Brazil at all, but were inferior Indian 
stones shipped to Brazil from Goa. The tables 
were neatly turned when diamonds were actually 
shipped from Brazil to Goa, and exported thence to 
Europe as Indian stones. This course and the 
continuous development of the diamond district in 
Brazil rendered it impossible to hoodwink the world 
indefinitely. The drop in prices was, however, 
stayed by the action of the Portuguese government, 
who exacted such heavy duties and imposed such 
onerous conditions that finally no one would under- 
take to work the mines. Accordingly, in 1772 
diamond-mining was declared a royal monopoly in 
Brazil, and such it remained until the severance of 
Brazil from Portugal in 1834, when private mining 
was permitted by the new government subject to the 
payment of reasonable royalties. The industry was 
enormously stimulated by the discovery, in 1 844, of 



OCCURRENCE OF DIAMOND 139 

the remarkably rich fields in the State of Bahia, 
especially at Serra da Cincora, where carbonado, 
or black diamond, first came to light, but after a- 
few years, owing to the difficulties of supplying 
labour, the unhealthiness of the climate, and the 
high cost of living, the yield fell off and gradually 
declined, until the importance of the fields was finally 
eclipsed by the rise of the South African mines. 
The Brazilian mines have proved very productive, 
but chiefly in small diamonds, stones above a carat 
in weight being few in comparison. The largest 
stone, to which the name, the Star of the South, 
was applied, weighed in the rough 254^ carats; it 
was discovered at the Bagagem mines in 1853. 
The quality of the diamonds is good, many of them 
having the highly-prized bluish-white colour. The 
principal diamond-bearing districts of Brazil centre 
at Diamantina, as Tejuco was re-named after the 
discovery of diamonds, Grao Magor, and Bagagem 
in the State of Minas Geraes, at Diamantina in the 
State of Bahia, and at Goyaz and Matto Grosso in 
the States of the same names. The diamonds 
occur chiefly in cascalho, a gravel, containing large 
masses of quartz and small particles of gold, which 
is supposed to be derived from a quartzose variety 
of micaceous slate known as itacolumite. The 
mines are now to some extent being worked by 
systematic dredging of the river-beds. 

Early in 1867 the children of a Boer farmer, 
Daniel Jacobs, who dwelt near Hopetown on the 
banks of the Orange River, picked up in the course 
of play near the river a white pebble, which was 
destined not only to mark the commencement of a 
new epoch in the record of diamond mines, but to 



140 GEM-STONES 

change the whole course of the history of South 
Africa. This pebble attracted the attention of a 
neighbour, Schalk van Niekerk, who suspected that 
it might be of some value, and offered to buy it. 
Mrs. Jacobs, however, gave it him, laughingly scout- 
ing the idea of accepting money for a mere pebble. 
Van Niekerk showed it to a travelling trader, by 
name John O'Reilly, who undertook to obtain what 
he could for it on condition that they shared the 
proceeds. Every one he met laughed to scorn the 
idea that the stone had any value, and it was once 
thrown away and only recovered after some search 
in a yard, but at length he showed it to Lorenzo 
Boyes, the Acting Civil Commissioner at Colesberg, 
who, from its extreme hardness, thought it might be 
diamond and sent it to the mineralogist, W. Guybon 
Atherston, of Grahamstown, for determination. So 
uncertain was Boyes of its value that he did not 
even seal up the envelope containing it, much less 
register the package. Atherston found immediately 
that the long-scorned pebble was really a fine 
diamond, weighing 21^- carats, and with O'Reilly's 
consent he submitted it to Sir Philip Wodehouse, 
Governor at the Cape. The latter purchased it at 
once for 500, and dispatched it to be shown at 
the Paris Exhibition of that year. It did not, 
however, attract much attention ; chimerical tales 
of diamond finds in remote parts of the world are 
not unknown. Indeed, for some time only a few 
small stones were picked up beside the Orange 
River, and no one believed in the existence of any 
extensive diamond deposit. However, all doubt as 
to the advisibility of prospecting the district was 
settled by the discovery of the superb diamond, 



PLATE 




-. 



PLATE XVI 







OCCURRENCE OF DIAMOND 141 

afterwards known as the ' Star "of South Africa,' 
which was picked up in March 1869 by a shepherd 
boy on the Zendfontein farm near the Orange River. 
Van Niekerk, on the alert for news of further dis- 
coveries, at once hurried to the spot and purchased 
the stone from the boy for five hundred sheep, ten 
oxen, and a horse, which seemed to the boy untold 
wealth, but was not a tithe of the 11,200 which 
Lilienfeld Bros., of Hopetown, gave Van Niekerk. 

This remarkable discovery attracted immediate 
attention to the potentialities of a country which 
produced diamonds of such a size, and prospectors 
began to swarm into the district, gradually spread- 
ing up the Vaal River. For some little time not 
much success was experienced, but at length, early 
in 1870, a rich find was made at Klipdrift, now 
known as Barkly West, which was on the banks of 
the Vaal River immediately opposite the Mission 
camp at Pniel. The number of miners steadily 
increased until the population on the two sides of 
the river included altogether some four or five 
thousand people, and there was every appearance 
of stability in the existing order of things. But a 
vast change came over the scene upon the discovery 
of still richer mines lying to the south-east and some 
distance from the river. The ground was actually 
situated on the route traversed by parties hurrying 
to the Vaal River, but no one dreamed of the wealth 
that lay under their feet. The first discovery was 
made in August 1870 at the farm Jagersfontein, 
near Fauresmith in Orange River Colony, by De 
Klerk, the intelligent overseer, who noticed in the dry 
bed of a stream a number of garnets, and, knowing 
that they often accompanied diamond, had the curi- 



142 GEM-STONES 

osity to investigate the point. He was immediately 
rewarded by finding a fine diamond weighing 50 
carats. In the following month diamonds were 
discovered about twenty miles from Klipdrift at 
Dutoitspan on the Dorstfontein farm, and a little 
later also on the contiguous farm of Bultfontein ; a 
diamond was actually found in the mortar used in 
the homestead of the latter farm. Early in May 
1871 diamonds were found about two miles away 
on De Beers' farm, Vooruitzigt, and two months 
later, in July, a far richer find was made on the 
same farm at a spot which was first named Coles- 
berg Kopje, the initial band of prospectors having 
come from the town of that name near the Orange 
River, but was subsequently known as Kimberley 
after the Secretary of State for the Colonies at that 
time. Soon a large and prosperous town sprang 
up close to the mines ; it rapidly grew in size and 
importance, and to this day remains the centre of 
the diamond-mining industry. Subsequent pro- 
specting proved almost blank until the discovery 
of the Premier or Wesselton mine on Wesselton 
farm, about four miles from Kimberley, in September 
1890; it received the former name after Rhodes, 
who was Premier of Cape Colony at that date. No 
further discovery of any importance was made until, 
in 1902, diamonds were found about twenty miles 
north-west-north of Pretoria in the Transvaal, at the 
new Premier mine, now famed as the producer of 
the gigantic Cullinan diamond. 

The Kimberley mines were at first known as the 
' dry diggings ' on account of their arid surround- 
ings in contradistinction to the 'river diggings ' by 
the Vaal. The dearth of water was at first one of 



PLATE Xl'lII 



_o&jJia*-*L ** ? >d ,^/ i "*_'&'* *.! 




KIMBERLEY MINE, l83l 



OCCURRENCE OF DIAMOND 143 

the great difficulties in the way of working the 
former mines, although subsequently the accumula- 
tion of underground water at lower levels proved a 
great obstacle to the working of the mines. The 
' river diggings ' were of a type similar to that met 
with in India and Brazil, the diamonds occurring 
in a gravelly deposit of limited thickness beneath 
which was barren rock, but the Kimberley mines 
presented a phenomenon hitherto without precedent 
in the whole history of diamond mining. The 
diamonds were found in a loose surface deposit, 
which was easily worked, and for some time the 
prospectors thought that the underlying limestone 
corresponded to the bedrock of the river gravel, 
until at length one more curious than his fellows 
investigated the yellowish ground underneath, and 
found to his surprise that it was even richer than 
the surface layer. Immediately a rush was made 
back to the deserted claims, and the mines were 
busier than ever. This ' yellow ground,' as it is 
popularly called, was much decomposed and easy, 
therefore, to work and sift. About fifty to sixty 
feet (1518 m.) below the surface, however, it 
passed into a far harder rock, which from its colour 
is known as the ' blue ground ' ; this also, to the 
unexpected pleasure of the miners, turned out to 
contain diamonds. Difficulties arose as each claim, 
30 by 30 Dutch feet (about 31 English feet or 
9-45 metres square) in area, was worked downwards. 
In the Kimberley mine (Plate XVI) access to the vari- 
ous claims was secured by retaining parallel strips, 
i 5 feet wide, each claim being, therefore, reduced in 
width to 22| feet, to form roadways running from 
side to side of the mine in one direction. These, 



144 GEM-STONES 

however, soon gave way, not only because of the 
falling of the earth composing them, but because 
they were undermined and undercut by the owners 
of the adjacent claims. By the end of 1872 the 
last roadway had disappeared, and the mine pre- 
sented the appearance of a vast pit. In order to 
obtain access to the claims without intruding on 
those lying between, and to provide for the hauling 
of the loads of earth to the surface, an ingenious 
system of wire cables in three tiers (Plate XVII) was 
erected, the lowest tier being connected to the outer- 
most claims, the second to claims farther from the 
edge, and the highest to claims in the centre of 
the pit. The mine at that date presented a most 
remarkable spectacle, resembling an enormous 
radiating cobweb, which had a weird charm by 
night as the moonlight softly illuminated it, and by 
day, owing to the perpetual ring of the flanged 
wheels of the trucks on the running wires, twanged 
like some gigantic aeolian harp. This system ful- 
filled its purpose admirably until, with increasing 
depth of the workings, other serious difficulties arose. 
Deprived of the support of the hard blue ground, 
the walls of the mine tended to collapse, and 
additional trouble was caused by the underground 
water that percolated into the mine. By the end of 
1883 the floor of the Kimberley mine was almost 
entirely covered by falls of reef (Plate XVIII), as the 
surrounding rocks are termed, the depth then being 
about 400 feet (122 m.). In the De Beers mine, in 
spite of the precaution taken to prevent falls of reef 
by cutting the walls of the mine back in terraces, falls 
occurred continuously in 1884, and by 1887, at a 
depth of 350 feet (107 m.), all attempts at open work- 



PLATE XIX 




PLATE XX 




OCCURRENCE OF DIAMOND 145 

ing had to be abandoned. In the Dutoitspan mine 
buttresses of blue ground were left, which held back 
the reef for some years, but ultimately the mine 
became unsafe, and in March 1886 a disastrous 
fall took place, in which eighteen miners eight 
white men and ten Kafirs lost their lives. The 
Bultfontein mine was worked to the great depth of 
500 feet (152 m.), but falls occurred in 1889 and put 
an end to open working. In all cases, therefore, the 
ultimate end was the same : the floor of the mine 
became covered with a mass of worthless reef, which 
rendered mining from above ground dangerous, 
and, indeed, impossible except at prohibitive cost. 
It was then clearly necessary to effect access to 
the diamond-bearing ground by means of shafts 
sunk at a sufficient distance from the mine to re- 
move any fear of falls of reef. For such schemes 
co-operative working was absolutely essential. Plate 
XIX illustrates the desolate character of the Kimber- 
ley mine above ground and the vastness of the 
yawning pit, which is over 1000 feet (300 m.) in 
depth. 

A certain amount of linking up of claims had 
already taken place, but, although many men must 
have seen that the complete amalgamation of the 
interests in each mine was imperative, two men 
alone had the capacity to bring their ideas to 
fruition. C. J. Rhodes was the principal agent in 
the formation in April 1880 of the De Beers 
Mining Company, which rapidly absorbed the re- 
maining claims in the mine, and was re-formed in 
1887 as tne De Beers Consolidated Mining Com- 
pany. Meantime, Barnett Isaacs, better known by 
the cognomen Barnato, which had been adopted by his 
10 



146 GEM-STONES 

brother Henry when engaged in earning his livelihood 
in the diamond fields as an entertainer, had secured 
the major interests in the Kimberley mine. Rhodes 
saw that, for effective working of the two mines by 
any system of underground working, they must be 
under one management, but to all suggestions of 
amalgamation Barnato remained deaf, and at last 
Rhodes determined to secure control of the Kim- 
berley mine at all costs. The story of the titanic 
struggle between these two men forms one of the 
epics of finance. Eventually, when shares in the 
Kimberley mine had been boomed to an extra- 
ordinary height, and the price of diamonds had 
fallen as low as i8s. a carat, Barnato gave way, and 
in July 1889 the Kimberley mine was absorbed by 
the De Beers Company on payment of the enor- 
mous sum of 5, 3 3 8,6 50. Shortly afterwards they 
undertook the working of the Dutoitspan and the 
Bultfontein mines, and in January 1896 they 
acquired the Premier or Wesselton mine. The 
interests in the Jagersfontein mine were in 1888 
united in the New Jagersfontein Mining and Ex- 
ploration Company, and the mine is now worked also 
by the De Beers Company. Thus, until the develop- 
ment of the new Premier mine in the Transvaal, the 
De Beers Company practically controlled the diamond 
market. The development of this last mine was 
begun so recently, and its size is so vast the 
longest diameter being half a mile that open-cut 
working is likely to continue for some years. 

Though varying slightly in details, the methods of 
working the mines are identical in principle. From 
the steeply inclined shaft horizontal galleries are 
run diagonally right across the mine, the vertical 



PLATE XXI 





LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT OVER 



PLATE XXII 




OCCURRENCE OF DIAMOND 147 

interval between successive galleries being 40 feet. 
From each gallery side galleries are run at right 
angles to it and parallel to the working face. The 
blue ground is worked systematically backwards 
from the working face. The mass is stoped, i.e. 
drilled and broken from the bottom upwards, until 
only a thin roof is left. As soon as the section is 
worked out and the material removed, the roof is 
allowed to fall in, and work is begun on the next 
section of the same level; at the same time the 
first section on the level next below is opened out. 
Thus work is simultaneously carried on in several 
levels, and a vertical plane would intersect the 
working faces in a straight line obliquely inclined to 
the vertical direction (Fig. 60). When freshly mined, 
the blue ground is hard and compact, but it soon dis- 
integrates under atmospheric influence. Indeed, the 
yellow ground itself was merely decomposed blue 
ground. No immediate attempt is made, therefore, 
to retrieve the precious stones. The blue ground is 
spread on to the 'floors' (Plate XXI), i.e. spaces of 
open veldt which have been cleared of bushes and 
inequalities, to the depth of a couple of feet, and 
remains there for periods ranging from six months 
to two years, depending on the quality of the blue 
ground and the amount of rainfall. To hasten 
the disintegration the blue ground is frequently 
ploughed over and occasionally watered, a remark- 
able introduction of agricultural methods into mining 
operations. No elaborate patrolling or guarding is 
required, because the diamonds are so sparsely, 
though regularly, scattered through the mass that 
even of the actual workers in the mines but few have 
ever seen a stone in the blue ground. When 



148 GEM-STONES 

sufficiently broken up, it is carted to the washing 
and concentrating machines, by means of which the 
diamonds and the heavier constituents are separated 
from the lighter material. 




FlG. 60. Vertical Section of Diamond Pipe, showing Tunnels and Slopes. 

Formerly the diamonds were picked out from the 
concentrates by means of the keen eyes of skilled 
natives ; but the process has been vastly simplified 
and the risk of theft entirely eliminated by the 
remarkable discovery made in 1897 by F. Kirsten, 



PLATE XXI 11 




PLATE XX II' 




OCCURRENCE OF DIAMOND 149 

of the De Beers Company, that of all the heavy 
constituents of the blue ground diamond alone, with 
the exception of an occasional corundum and zircon, 
which are easily sorted out afterwards, adheres to 
grease more readily than to water. In this 
ingenious machine, the 'jigger ' or 'greaser' (Plate 
XXIII) as it is commonly termed, the concentrates are 
washed over a series of galvanized-iron trays, which 
are covered with a thick coat of grease. The trays 
are slightly inclined downwards, and are kept by 
machinery in constant sideways motion backwards 
and forwards. So accurate is the working of this 
device that few diamonds succeed in getting beyond 
the first tray, and none progress as far as the third, 
which is added as an additional precaution. The 
whole apparatus is securely covered in so that there 
is no risk of theft during the operation. The trays 
are periodically removed, and the grease is scraped 
off and boiled to release the diamonds, the grease 
itself being used over again on the trays. This is 
the first time in the whole course of extraction from 
the mines that the diamonds are actually handled. 
The stones are now passed on to the sorters, who 
separate them into parcels according to their size, 
shape, and quality. 

The classification at the mines is first into groups 
by the shape: (i) close goods, (2) spotted stones, 
(3) rejection cleavage, (4) fine cleavage, (5) light 
brown cleavage, (6) ordinary and rejection cleavage, 
(7) flats, (8) macles, (9) rubbish, (10) boart. Close 
goods are whole crystals which contain no flaws and 
can be cut into single stones. Spotted stones, as 
their name suggests, contain spots which necessitate 
removal, and cleavage includes stones which are so 



ISO GEM-STONES 

full of flaws that they have to be cleaved or split 
into two or more stones. Flats are distorted 
octahedra, and macles are twinned octahedra. 
Rubbish is material which can be utilized only for 
grinding purposes, and boart consists of round dark 
stones which are invaluable for rock-drills. These 
groups are afterwards graded into the following 
subdivisions, depending on increasing depth of 
yellowish tint : (a) blue-white, () first Cape, (c) 
second Cape, (d) first bye, (e) second bye, (/) off- 
colour, (g) light yellow, (ft) yellow. It is, however, only 
the first group that is so minutely subdivided. After 
being purchased, the parcels are split up again some- 
what differently for the London market (cf. p. 136), 
and the dealers re-arrange the stones according to 
the purpose for which they are required. Formerly 
a syndicate of London merchants took the whole of 
the produce of the Kimberley mines at a previously 
arranged price per carat, but at the present time 
the diamonds are sold by certain London firms on 
commission. 

The products of each mine show differences in 
either form or colour which enable an expert readily 
to recognize their origin. The old diggings by the 
Vaal River yielded finer and more colourless stones 
than those found in the dry diggings and the mines 
underlying them. The South African diamonds, 
taken as a whole, are always slightly yellowish or 
' off-coloured ' ; the mines are, indeed, remarkable 
for the number of fine and large, canary-yellow and 
brown, stones produced. The Kimberley mine 
yields a fair percentage of white, and a large number 
of twinned and yellow stones. The yield of the De 
Beers mine comprises mostly tinted stones yellow 



OCCURRENCE OF DIAMOND 151 

and brown, occasionally silver capes, and very 
seldom stones free from colour. The Dutoitspan 
mine is noted for its harvest of large yellow 
diamonds ; it also produces fine white cleavage and 
small white octahedra. The stones found in the 
Bultfontein mine are small and spotted, but, on the 
other hand, the yield has been unusually regular. 
The Premier or Wesselton mine yields a large pro- 
portion of flawless octahedra, but, above all, a large 
number of beautiful deep-orange diamonds. Of all 
the South African mines the Jagersfontein in the 
Orange River Colony alone supplies stones of 
the highly-prized blue-white colour and steely lustre 
characteristic of the old Indian stones. The new 
Premier mine in the Transvaal is prolific, but mostly 
in off-coloured and low-grade stones, the Cullinan 
diamond being a remarkable exception. 

To illustrate the amazing productiveness of the 
South African mines, it may be mentioned that, 
according to Gardner F. Williams, the Kimberley 
group of mines in sixteen years yielded 36 million 
carats of diamonds, and the annual output of the 
Jagersfontein mine averages about a quarter of a 
million carats, whereas the total output of the Brazil 
mines, for the whole of the long period during which 
they have been worked, barely exceeds 13 million 
carats. The average yield of the South African 
mines, however, perceptibly diminishes as the depth 
of the mines increases. 

The most interesting point connected with the 
South African diamond mines, viewed from the 
scientific standpoint, is the light that they have 
thrown on the question of the origin of the diamond, 
which previously was an incomprehensible and 



1 5 2 GEM-STONES 

apparently insoluble problem. In the older mines, 
just as at the river diggings by the Vaal, the stones 
are found in a gravelly deposit that has resulted 
from the disintegration of the rocks through which 
the adjacent river has passed, and it is clear that 
the diamond cannot have been formed in situ here ; 
it had been suspected, and now there is no doubt, 
that the itacolumite rock of Brazil has consolidated 
round the diamonds which are scattered through it, 
and that it cannot be the parent rock. The 
occurrence at Kimberley is very different. These 
mines are funnels which go downwards to unknown 
depths; they are more or less oval in section, 
becoming narrower with increasing depth, and are 
evidently the result of some eruptive agency. The 
Kimberley mine has been worked to a depth of 
nearly 4000 feet (1200 m.), and no signs of a 
termination have as yet appeared. The blue ground 
which fills these ' pipes,' as they are termed, must 
have been forced up from below, since it is sharply 
differentiated from the surrounding country rocks. 
This blue ground is a brecciated peridotite of peculiar 
constitution, to which the well-known petrologist, 
Carvil Lewis, who made a careful study of it, gave 
the name kimberlite. The blue colour testifies to its 
richness in iron, and it is to the oxidation of the iron 
constituent, that the change of colour to yellow in 
the upper levels is due. Owing to the shafts that 
have been sunk for working the mines, the nature of 
the surrounding rocks is known to some depth. 
Immediately below the surface is a decomposed 
ferriferous basalt, about 20 to 90 feet (6-27 m.) 
thick, next a black slaty shale, 200 to 250 
feet (60-75 m.) thick, then 10 feet (3 m.) of 



OCCURRENCE OF DIAMOND 153 

conglomerate, next 400 feet (120 m.) of olivine 
diabase, then quartzite, about 400 feet (120 m.) 
thick, and lastly a quartz porphyry, which has 
not yet been penetrated. The strata run nearly 
horizontal, and there are no signs of upward 
bending at the pipes. The whole of the country, 
including the mines, was covered with a red sandy 
soil, and there was nothing to indicate the wealth 
that lay underneath. The action of water had in 
process of time removed all signs of eruptive activity. 
The principal minerals which are associated with 
diamond in the blue ground are magnetite, ilmenite, 
chromic pyrope, which is put on the market as a 
gem under the misnomer ' Cape-ruby,' ferriferous 
enstatite, which also is sometimes cut, olivine more 
or less decomposed, zircon, kyanite, and mica. 

The evidence produced by an examination of the 
blue ground and the walls of the pipes proves that 
the pipes cannot have been volcanoes such as 
Vesuvius. There is no indication whatever of the 
action of any excessive temperature, while, on the 
other hand, there is every sign of the operation of 
enormous pressure; the diamonds often contain 
liquid drops of carbonic acid. Crookes puts forward 
the plausible theory that steam has been the 
primary agency in propelling the diamond and its 
associates up into the channel through which it has 
carved its way to freedom, and holds that molten 
iron has been the solvent for carbon which has 
crystallized out as diamond under the enormous 
pressures obtaining in remote depths of the earth's 
crust. It is pertinent to note that, by dissolving 
carbon in molten iron, the eminent chemist, Moissan, 
was enabled to manufacture tiny diamond crystals. 



154 GEM-STONES 

Water trickling down from above would be im- 
mediately converted into steam at very high 
pressure on coming into contact with the molten 
iron, and, in its efforts to escape, the steam would 
drive the iron and its precious contents, together 
with the adjacent rocks, upwards to the surface. 
The ferriferous nature of the blue ground and the 
yellow tinge so common to the diamonds lend 
confirmation to this theory. The process by which 
the carbon was extracted from shales or other 
carboniferous rocks and dissolved in iron still awaits 
elucidation. 

Diamonds were found in New South Wales as long 
ago as 1851 on Turon River and at Reedy Creek, 
near Bathurst, about ninety miles (145 km.) from 
Sydney, but the find was of little commercial import- 
ance. A more extensive deposit came to light in 
1867 farther north at Mudgee. In 1872 diamonds 
were discovered in the extreme north of the State, 
at Bingara near the Queensland border. Another 
discovery was made in 1884 at Tingha, and still 
more recently in the tin gravels of Inverell in the 
same region. In their freedom from colour and 
absence of twinning the New South Wales diamonds 
resemble the Brazilian stones. The average size 
is small, running about five to the carat when cut; 
the largest found weighed nearly 6 carats when 
cut. They are remarkable for their excessive 
hardness; they can be cut only with their own 
dust, ordinary diamond dust making no impres- 
sion. 

The Borneo diamonds are likewise distinguished by 
their exceptional hardness. They mostly occur by 
the river Landak, near Pontianak on the west coast 



OCCURRENCE OF DIAMOND 155 

of the island. They are found in a layer of rather 
coarse gravel, variable, but rarely exceeding a yard 
(i m.), in depth, and are associated with corundum 
and rutile, together with the precious metals gold 
and platinum. Indeed, it is no uncommon sight to 
see natives wearing waistcoats ornamented with gold 
buttons, in each of which a diamond is set. The 
diamonds are well crystallized and generally of 
pure water ; yellowish and canary-yellow stones are 
also common, but rose-red, bluish, smoky, and 
black stones are rare. They seldom exceed a 
carat in weight ; but stones of I o carats in weight 
are found, and occasionally they attain to 20 
carats. In 1850 a diamond weighing 77 carats 
was discovered. The Rajah of Mattan is said 
to possess one of the purest water weighing as 
much as 367 carats, but no one qualified to pro- 
nounce an opinion regarding its genuineness has 
ever seen it. 

In Rhodesia small diamonds have been found 
in gravel beds resting on decomposed granite near 
the Somabula forest, about 12 miles (19 km.) west 
of Gwelo, in association with chrysoberyl in abund- 
ance, blue topaz, kyanite, ruby, sapphire, tourmaline, 
and garnet. 

The occurrence of diamond in German South- 
West Africa is very peculiar. Large numbers of 
small stones are found close to the shore near 
Luderitz Bay in a gravelly surface layer, which is 
nowhere more than a foot in depth. They are 
picked by hand by natives and washed in sieves. 
In shape they are generally six-faced octahedra 
or twinned octahedra, simple octahedra being rare, 
and in size they run about four or five to the 



156 GEM-STONES 

carat, the largest stone as yet found being only 
2 carats in weight. Their colour is usually 
yellowish. 

Several isolated finds of diamonds have been 
reported in California and other parts of the United 
States, but none have proved of any importance. 
The largest stone found weighed 23! carats uncut; 
it was discovered at Manchester in Virginia. 



CHAPTER XVIII 
HISTORICAL DIAMONDS 

THE number of diamonds which exceed a 
hundred carats in weight when cut is very 
limited. Their extreme costliness renders them 
something more than mere ornaments ; in a 
condensed and portable form they represent great 
wealth and all the potentiality for good or ill 
thereby entailed, and have played no small, if 
sinister, r61e in the moulding of history. In bygone 
days when despotic government was universal, the 
possession of a splendid jewel in weak hands but 
too often precipitated the aggression of a greedy 
and powerful neighbour, and plunged whole 
countries into the horrors of a ruthless and bloody 
war. In more civilized days a great diamond has 
often been pledged as security for money to 
replenish an empty treasury in times of stress. 
The ambitions of Napoleon might have received 
a set-back but for the funds raised on the security 
of the famous Pitt diamond. The history of 
such stones often one long romance is full of 
interest, but space will not permit of more than 
a brief sketch here. 

If we except the colossal Cullinan stone, the 
mines of Brazil and South Africa cannot compare 
with the old mines of India as the birthplace of 
large and perfect diamonds of world-wide fame. 
157 



158 



GEM-STONES 




FIG. 61. Koh-i-nor (top 
view). 



(l) KOH-I-NOR 

The history of the famous stone called the 
Koh-i-nor, meaning Mound of Light, is known as 
far back as the year 1304, when it fell into the 
hands of the Mogul em- 
perors, and legend even 
traces it back some four 
thousand years previously. 
It remained at Delhi until 
the invasion of North- West 
India by Nadir Shah in 
1739, when it passed to- 
gether with an immense 
amount of spoil into the 
hands of the conqueror. 
At his death the empire which he had so strenu- 
ously founded fell to pieces, and the great diamond 
after many vicissitudes came into the possession of 
Runjit Singh at Lahore. His successors kept it 
until upon the fall of the 
Sikh power in 1850 it 
passed to the East India 
Company, in whose name 
it was presented by Lord 
Dalhousie to Queen Victoria. 
At this date the stone still 
retained its original Indian 

form, but in 1862 it was re-cut into the form of 
a shallow brilliant (Fig. 62), the weight thereby 
being reduced from i86 T V to io6 T V carats. The 
wisdom of this course has been severely criticized ; 
the stone has not the correct shape of a brilliant 
and is deficient in ' fire,' and it has with the change 




FIG. 62. Koh-i-nor (side 
view). 



HISTORICAL DIAMONDS 



159 



in shape lost much of its old historical interest. 
The Koh-i-nor is the private poperty of the English 
Royal Family, the stone shown in the Tower being 
a model. It is valued at ,100,000. 




FIG. 63. Pitt or Regent 
(top view). 



(2) PITT OR REGENT 

This splendid stone was discovered in 1701 at 

the famous diamond mines 

at Partial, on the Kistna, 

about 150 miles (240 km.) 

from Golconda, and weighed 

as much as 410 carats in the 

rough. By devious ways it 

came into the hands of Jam- 

chund, a Parsee merchant, 

from whom it was purchased 

by William Pitt, governor of 

Fort St. George, Madras, for 

20,400. On his return to England Pitt had it cut 

into a perfect brilliant (Fig. 63), weighing 163^ 

carats, the operation ocupying the space of two 
years and costing 5000 ; more 
than 7000 is said to have 
been realized from the sale of 
the fragments left over. Pitt 
had an uneasy time and lived 
in constant dread of theft of the 

Reent stone until> in 1 7 1 7> after len s th y 

negotiations, he parted with it to 
the Due d'Orleans, Regent of France, for the immense 
sum of three and three-quarter million francs, about 
,1 3 5,000. With the remainder of the French regalia 
it was stolen from the Garde-meuble on August r7, 




160 GEM-STONES 

1792, in the early days of the French Revolution, 
but was eventually restored by the thieves, doubt- 
less because of the impossibility of disposing of such 
a stone, at least intact, and it is now exhibited 
in the Apollo Gallery of the Louvre at Paris. It 
measures about 30 millimetres in length, 25 in width, 
and 19 in depth, and is valued at ^480,000. 

(3) ORLOFF 

One of the finest diamonds existing, this large 
stone forms the top of 
the imperial sceptre of 




FIG. 65. Orloff (top view). FIG. 66. Orloff (side view). 

Russia. It is rose-cut (Fig. 65), the base being a 
cleavage face, and weighs i94f carats. It is said 
to have formed at one time one of the eyes of a 
statue of Brahma which stood in a temple on the 
island of Sheringham in the Cavery River, near 
Trichinopoli, in Mysore, and to have been stolen 
by a French soldier who had somehow persuaded 
the priests to appoint him guardian of the temple. 
He sold it for 2000 to the captain of an English 
ship, who disposed of it to a Jewish dealer in 
London for ,12,000. It changed hands to a 
Persian merchant, Raphael Khojeh, who eventually 
sold it to Prince Orloff for, so it is said, the immense 



HISTORICAL DIAMONDS 161 

sum of 90,000 and an annuity of 4000. It 
was presented by Prince Orloff to Catherine II of 
Russia. 

(4) GREAT MOGUL 

This, the largest Indian diamond known, was 
found in the Kollur mines, about the year 1650. 
Its original weight is said to have been 787^ carats, 
but it was so full of flaws that the Venetian, Hortensio 
Borgis, then in India, in cutting it to a rose form 
reduced its weight to 240 carats. It was seen by 
Tavernier at the time of his visit to India, but it 
has since been quite lost sight of. It has been 
identified with both the Koh-i-nor and the Orloff, 
and it is even suggested that both these stones were 
cut from it. 

(5) SANCY 

The history of this diamond is very involved, and 
probably two or more stones have been confused. 
It may have been the one cut by Berquem for 
Charles the Bold, from whose body on the fatal day 
of Nancy, in 1477, it was snatched by a marauding 
soldier. It was acquired by Nicholas Harlai, 
Seigneur de Sancy, who sold it to Queen Elizabeth 
at the close of the sixteenth century. A hundred 
years later, in 1695, it was sold by James II to 
Louis XIV. The stone in the French regalia, 
according to the inventory taken in 1791, weighed 
53! carats. It was never recovered after the theft 
of the regalia in the following year, but may be 
identical with the diamond which was in the posses- 
sion of the Demidoff family and was sold by Prince 
Demidoff in 1865 to a London firm who were said 



1 62 GEM-STONES 

to have been acting for Sir Jamsetjee Jeejeebhoy, 
a wealthy Parsee of Bombay. It was shown at the 
Paris Exhibition of 1867. It was almond-shaped, 
and covered all over with tiny facets by Indian 
lapidaries. 

(6) GREAT TABLE 

This mysterious stone was seen by Tavernier at 
Golconda in 1642, but has quite disappeared. It 
weighed 242^ carats. 

(7) MOON OF THE MOUNTAINS 

This diamond is often confused with the Orloff. 
It was captured by Nadir Shah at Delhi, and after 
his murder was stolen by an Afghan soldier who 
disposed of it to an Armenian, by name Shaffrass. 
It was finally acquired by the Russian crown for an 
enormous sum. 

(8) NIZAM 

A large diamond, weighing 340 carats, belonged 
to the Nizam of Hyderabad ; it was fractured at 
the beginning of the Indian Mutiny. Whether the 
weight is that previous to fracture or not, there 
seems to be no information. 

(9) DARYA-I-NOR 

This fine diamond, rose-cut and 186 carats in 
weight, is of the purest water and merits its title of 
' River of Light.' It seems to have been captured 
by Nadir Shah at Delhi, and is now the largest 
diamond in the Persian collection. 



HISTORICAL DIAMONDS 163 



(10) SHAH 

This fine stone, of the purest water, was pre- 
sented to the Czar Nicholas by the Persian prince 
Chosroes, younger son of Abbas Mirza, in 1843. 
At that time it still retained three cleavage faces 
which were engraved with the names of three 
Persian sovereigns, and weighed 95 carats. It was, 
however, subsequently re-cut with the loss of 9 
carats, and the engraving has disappeared in the 
process. 

(n) AKBAR SHAH, OR JEHAN GHIR SHAH 

Once the property of the great Mogul, Akbar, this 
diamond was engraved on two faces with Arabic 
inscriptions by the instructions of his successor, 
Jehan. It disappeared, but turned up again in 
Turkey under the name of ' Shepherd's Stone ' ; it 
still retained its original inscriptions and was there- 
by recognized. In 1866 it was re-cut, the weight 
being reduced from 116 to 71 carats, and the in- 
scriptions destroyed. The stone was sold to the 
Gaekwar of Baroda for 3^ lakhs of rupees (about 
23,333). 

(12) POLAR STAR 

A beautiful, brilliant-cut stone, weighing 40 
carats, which is known by this name, is in the 
Russian regalia. 

(13) NASSAK 

The Nassak diamond, which weighed 89! carats, 
formed part of the Deccan booty, and was put up 



1 64 GEM-STONES 

to auction in London in July 1837. It was pur- 
chased by Emanuel, a London jeweller, who for 
7200 shortly afterwards sold it to the Duke of 
Westminster, in whose family it still remains. It 
was originally pear-shaped, but was re-cut to a 
triangular form with a reduction in weight to 78f 
carats. 

(14) NAPOLEON 

This diamond was purchased by Napoleon 
Buonaparte for 8000, and worn by him at his 
wedding with Josephine Beauharnais in 1796. 

(15) CUMBERLAND 

This stone, which weighs 32 carats, was purchased 
by the city of London for 10,000 and presented to 
the Duke of Cumberland after the battle of Culloden ; 
it is now in the possession of the Duke of Brunswick. 

(16) PlGOTT 

A fine Indian stone, weighing 47^ carats, this 
diamond was brought to England by Lord Pigott in 
1775 and sold for 30,000. It came into the 
possession of Ali Pacha, Viceroy of Egypt, and was 
by his orders destroyed at his death. 

(17) EUGENIE 

This fine stone, weighing 5 I carats, was given by 
the Czarina Catherine II of Russia to her favourite, 
Potemkin. It was purchased by Napoleon in as a 
bridal gift for his bride, and on his downfall was 
bought by the Gaekwar of Baroda. 



HISTORICAL DIAMONDS 165 

(18) WHITE SAXON 

Square in contour, measuring I ^ in. (28 mm.), 
and weighing 48! carats, this stone was purchased 
by Augustus the Strong for a million thalers (about 
150,000). 

(19) PACHA OF EGYPT 

This 4O-carat brilliant was purchased by Ibrahim, 
Viceroy of Egypt, for 28,000. 

(20) STAR OF ESTE 

Though a comparatively small stone, in weight 
25^ carats, it is noted for its perfection of form and 
quality. It belongs to the Archduke Franz Ferdi- 
nand of Austrian-Este, eldest son of the Archduke 
Karl Ludwig. 

(21) TUSCANY, OR AUSTRIAN YELLOW 

The beauty of this large stone, 133! carats in 
weight, is marred by the tinge of yellow, which is 
sufficiently pronounced to impair its brilliancy ; 
it is a double rose in form. At one time the 
property of the Grand Dukes of Tuscany, it is now 
in the possession of the Emperor of Austria. King 
mentions a tale that it was bought at a curiosity 
stall in Florence for an insignificant sum, the stone 
being supposed to be only yellow quartz. 

(22) STAR OF THE SOUTH 

This, the largest of the Brazilian diamonds, was 
discovered at the mines of Bagagem in July 1853. 



1 66 GEM-STONES 

Perfectly transparent and without tint, it was 
dodecahedral in shape and weighed 254! carats, 
and was sold in the rough for 40,000. It was cut 
as a perfect brilliant, being reduced in weight to 
125^ carats. 

(23) ENGLISH DRESDEN 

This beautiful stone, which weighed 1 1 9^ carats 
in the rough, was found at the Bagagem mines, in 
Brazil, in 1857, a d came into the possession of 
Mr. E. Dresden. It was cut as a long, egg-shaped 
brilliant, weighing 76^ carats. 

(24) STAR OF SOUTH AFRICA 

The first considerable stone to be found in South 
Africa, it was discovered at the Vaal River diggings 
in 1869, and weighed 83^ carats in the rough. It 
was cut to a triangular brilliant of 46^ carats. 
It was finally purchased by the Countess of Dudley 
for 25,000. 

(25) STEWART 

This large diamond, weighing in the rough 288f 
carats, was found at the Vaal River diggings in 
1872, and was first sold for 6000 and shortly 
afterwards for 9000 ; it was reduced on cutting to 
1 20 carats. Like many South African stones, it 
has a faint yellowish tinge. 

(26) PORTER-RHODES 

This blue- white stone, which weighed 150 carats, 
was found in a claim belonging to Mr. Porter- 
Rhodes in the Kimberley mine in February 1880. 



HISTORICAL DIAMONDS 167 

(27) IMPERIAL, VICTORIA, OR GREAT WHITE 

This large diamond weighed as much as 457 
carats in the rough, and 180 when cut; it is quite 
colourless. It was brought to Europe in 1884, an d 
was eventually sold to the Nizam of Hyderabad 
for 20,000. 

(28) DE BEERS 

A pale yellowish stone, weighing 42 8 \ carats, 
was found in the De Beers mine in 1888. It was 
cut to a brilliant weighing 22 8 1 carats, and was sold 
to an Indian prince. A still larger stone of similar 
tinge, weighing 503^ carats, was discovered in 1896, 
and among other large stones supplied by the same 
mine may be mentioned one of 302 carats found in 
1 884, and another of 409 carats found in early years. 

(29) EXCELSIOR 

This, which prior to the discovery of the ' Cullinan,' 
was by far the largest South African stone, was 
found in the Jagersfontein mine on June 30, 1893 j 
bluish-white in tint, it weighed 969^ carats. From 
it were cut twenty-one brilliants, the larger stones 
weighing 67!, 45U, 45H, 39&, 34, 27$, 25!, 23^, 
I &r|, 1 3 carats respectively, and the total weight 
of the cut stones amounting to 364^% carats. 

(30) JUBILEE 

Another large stone was discovered in the 
Jagersfontein mine in 1895. It weighed 634 
carats in the rough, and from it was obtained a 
splendid, faultless brilliant weighing 239 carats. It 
was shown at the Paris Exhibition of 1900, 



i68 



GEM-STONES 



(31) STAR OF AFRICA, OR CULLINAN 

All diamonds pale into insignificance when com- 
pared with the colossal stone that came to light at 
the Premier mine near Pretoria in the Transvaal on 
January 25, 1905. It was first called the 
' Cullinan ' after Sir T. M. Cullinan, chairman of the 
Premier Diamond Mine (Transvaal) Company, but 
has recently, by desire 
of King George V, re- 
ceived the name ' Star 
of Africa.' The rough 
stone weighed 621-2 
grams or 3025!- carats 
(about i lb.); it dis- 
played three natural 
faces (Plate XXV) and 
one large cleavage face, 
and its shape suggested 
that it was a portion 
of an enormous stone 
more than double its 
size ; it was trans- 
parent, colourless, and 
had only one small flaw near the surface. This 
magnificent diamond was purchased by the Trans- 
vaal Government for 150,000, and presented to 
King Edward VII on his birthday, November 9, 1907. 
The Cullinan was entrusted to the famous firm, 
Messrs. I. J. Asscher & Co., of Amsterdam, for 
cutting on January 23, 1908, just three years after 
its discovery. On February 10 it was cleaved into 
two parts, weighing respectively 1977^ and 1040^ 
carats, from which the two largest stones have been 




FIG. 67. Cullinan No. 



PLATE XXV 




HISTORICAL DIAMONDS 



169 



cut, one being a pendeloque or drop brilliant in 
shape (Fig. 67) and weighing 5 1 6% carats, and the 
other a square brilliant (Fig. 68) weighing 3O9 T V 
carats. The first has been placed in the sceptre, and 
the second in the 
crown of the regalia. 
Besides these there 
are a pendeloque 
weighing 92 carats, a 
square-shaped brilli- 
ant 62, a heart-shaped 
stone 1 8 1 , two mar- 
quises 8 T V and 1 1 J, 
an oblong stone 6|, 
a pendeloque 4-^-, 
and 96 small brill- 
iants weighing to- 
gether 7 1 ; the total weight of the cut stones 
amounts to 1036/0 carats. The largest stone has 
74 and the second 66 facets. The work was 
completed and the stones handed to King Edward 
in November 1908. 

Although the Premier mine has yielded no worthy 
compeer of the Cullinan, it can, nevertheless, boast 
of a considerable number of large stones which but 
for comparison with that giant would be thought 
remarkable for their size, no fewer than seven of 
them having weights of over 300 carats, viz. 511, 
, 45 8f, 39ii, 373. 348, and 334 carats. 




FIG. 68. Cullinan No. 2. 



(32) STAR OF MINAS 

This large diamond, which was found in 1911 at 
the Bagagem mines, Minas Geraes, Brazil, had the 



1 70 GEM-STONES 

shape of a dome with a flat base, and weighed in 
the rough 3 5 '8 7 5 grams (174! carats). 



The large stone called the ' Braganza,' in the 
Portuguese regalia, which is supposed to be a 
diamond, is probably a white topaz ; it weighs 
1680 carats. The Mattan stone, pear-shaped and 
weighing 367 carats, which was found in the Landak 
mines near the west coast ot Borneo in 1787, is 
suspected to be quartz. 

COLOURED DIAMONDS 

(i) HOPE 

The largest of coloured diamonds, the Hope, 
weighs 44^ carats, and has a steely- or greenish- 
blue, and not the royal-blue colour of the glass 
models supposed to represent it. It is believed to 
be a portion of a drop- form stone 
(d'un beau violet} which was said 
to have been found at the Kollur 
mines, and was secured by Taver- 
nier in India in 1642 and sold 
by him to Louis XIV in 1668; 
FIG. 6 9 .-Hope. * then weighed 67 carats. This 
stone was stolen with the re- 
mainder of the French regalia in 1792 and never 
recovered. In 1830 the present stone (Fig. 69) 
was offered for sale by Eliason, a London dealer, 
and was purchased for 18,000 by Thomas Philip 
Hope, a wealthy banker and a keen collector of 
gems. Probably the apex of the original stone 
had been cut off, reducing it to a nearly square 




HISTORICAL DIAMONDS 171 

stone. The slight want of symmetry of the present 
stone lends confirmation to this view, and two other 
blue stones are known, which, together with 'the 
Hope, make up the weight of the original stone. 
At the sale of the Hope collection at Christie's in 
1867 the blue diamond went to America. In 1908 
the owner disposed of it to Habib Bey for the 
enormous sum of 80,000. It was put up to 
auction in Paris in 1909, and bought by Rosenau, 
the Paris diamond merchant, for the comparatively 
small sum of 400,000 francs (about 16,000), and 
was sold in January 1911 to Mr. Edward M'Lean 
for 60,000. The stone is supposed to bring ill- 
luck in its train, and its history has been liberally 
embellished with fable to establish the saying. 

(2) DRESDEN 

A beautiful apple-green diamond, faultless, and 
of the purest water, is contained in the famous 
Green Vaults of Dresden. It weighs 40 carats, and 
was purchased by Augustus the Strong in 1743 for 
60,000 thalers (about 9000). 

(3) PAUL I 

A fine ruby-red diamond, weighing 10 carats, is 
included among the Russian crown jewels. 

(4) TIFFANY 

The lovely orange brffliant, weighing 125! carats, 
which is in the possession of Messrs. Tiffany & Co., 
the well-known jewellers of New York, was dis- 
covered in the Kimberley mine in 1878. 



CHAPTER XIX 
CORUNDUM 

(Sapphire, Ruby} 

RANKING in hardness second to diamond 
alone, the species known to science as 
corundum and widely familiar by the names of its 
varieties, sapphire and ruby, holds a pre-eminent 
position among coloured gem-stones. The barbaric 
splendour of ruby (Plate I, Fig. 13) and the 
glorious hue of sapphire (Plate I, Fig. n) are 
unsurpassed, and it is remarkable that the same 
species should boast such different, but equally 
magnificent, tints. They, however, by no means 
exhaust the resources of this variegated species. 
Fine yellow stones (Plate I, Fig. 12), which compare 
with topaz in colour and are its superior in hard- 
ness, and brilliant colourless stones, which are 
unfortunately deficient in ' fire ' and cannot there- 
fore approach diamond, are to be met with, besides 
others of less attractive hues, purple, and yellowish, 
bluish, and other shades of green. Want of homo- 
geneity in the coloration of corundum is a frequent 
phenomenon ; thus, the purple stones on close 
examination are found to be composed of alternate 
blue and red layers, and stones showing patches of 

yellow and blue colour are common. Owing to the 
173 



CORUNDUM 173 

peculiarity of their interior arrangement certain 
stones display when cut en cabochon a vivid six- 
rayed star of light (Plate I, Fig. 15). Sapphire 
and ruby share with diamond, pearl, and emerald 
the first rank in jewellery. They are popular stones, 
especially in rings ; their comparative rarity in large 
sizes, apart from the question of expense, prevents 
their use in the bigger articles of jewellery. The 
front of the stones is usually brilliant-cut and the 
back step-cut, but Indian lapidaries often prefer to 
cover the stone with a large number of triangular 
facets, especially if the stone be flawed ; star-stones 
are cut more or less steeply en cabochon. 

In composition corundum is alumina, oxide of 
aluminium, corresponding to the formula A1 2 O 3 , but 
it usually contains in addition small quantities, 
rarely more than I per cent., of ferric oxide, chromic 
oxide, and perhaps other metallic oxides. When 
pure, it is colourless ; the splendid tints which are 
its glory have their origin in the minute traces of 
the other oxides present. No doubt chromic oxide 
is the cause of the ruddy hue of ruby, since it is 
possible, as explained above (p. 117), closely to 
imitate the ruby tint by this means, but nothing 
approaching so large a percentage as 2\ has been 
detected in a natural stone. The blue colour of 
sapphire may be due to titanic oxide, and ferric 
oxide may be responsible for the yellow hue of the 
' oriental topaz,' as the yellow corundum is termed. 
Sapphires, when of considerable size, are rarely 
uniform in tint throughout the stone. Alternations 
of blue and red zones, giving rise to an apparent 
purple or violet tint, and the conjunction of patches 
of blue and yellow are common. Perfectly colour- 



174 GEM-STONES 

less stones are less common, a slight bluish tinge 
being usually noticeable, but they are not in much 
demand because, on account of their lack of ' fire,' 
they are of little interest when cut. The tint of the 
red stones varies considerably in depth ; jewellers 
term them, when pale, pink sapphires, but, of course, 
no sharp distinction can be drawn between them and 
rubies. The most highly prized tint is the so-called 
pigeon's blood, a shade of red slightly inclined to 
purple. The prices for ruby of good colour run 
from about 253. a carat for small stones to between 
,60 and 80 a carat for large stones, and still 
higher for exceptional rubies. The taste in sapphires 
has changed of recent times. Formerly the deep 
blue was most in demand, but now the lighter shade, 
that resembling the colour of corn-flower, is preferred, 
because it retains a good colour in artificial light. 
Large sapphires are more plentiful than large rubies, 
and prices run lower ; even for large perfect stones 
the rate does not exceed 30 a carat. Large and 
uniform ' oriental topazes ' are comparatively 
common, and realize moderate prices, about 2s. to 
305. a carat according to quality and size. Green 
sapphires are abundant from Australia, but their 
tint, a kind of deep sage-green, is not very pleasing. 
Brown stones with a silkiness of structure are also 
known. 

The name of the species comes through the 
French corindon from an old Hindu word, korund, 
of unknown significance, and arose from the circum- 
stance that the stones which first found their way to 
Europe came from India. At the present day the 
word corundum is applied in commerce to the 
opaque stones used for abrasive purposes, to 'dis- 



CORUNDUM 175 

tinguish the purer material from emery, which is 
corundum mixed with magnetite and other heavy 
stones of lower hardness. The origin of the word 
sapphire, which means blue, has been discussed in an 
earlier chapter (p. 1 1 o). Jewellers use it in a 
general sense for all corundum except ruby. Ruby 
comes from the Latin ruber, red. The prefix 
' oriental ' (p. in) is often used to distinguish 
varieties of corundum, since it is the hardest of 
ordinary coloured stones and the finest gem-stones 
in early days reached Europe by way of the 
East. 

Corundum crystallizes either in six-sided prisms 
terminated by flat faces (Plate I, Fig. 10), which 
are often triangularly marked, or with twelve inclined 
faces, six above and six below, meeting in a girdle 
(Plate I, Fig. 14). Ruby favours the former and 
the other varieties the latter type. A fine crystal of 
ruby the ' Edwardes/ so named by the donor, John 
Ruskin, after Sir Herbert Edwardes which weighs 
33-5 grams (163 carats), is exhibited in the Mineral 
Gallery of the British Museum (Natural History), 
and is tilted in such a way that the light from a 
neighbouring window falls on the large basal face, 
and reveals the interesting markings that nature has 
engraved on it. From its type of symmetry corundum 
is doubly refractive with a direction of single refraction 
running parallel to the edge of the prism. Owing to 
the relative purity of the chemical composition the 
refractive indices are very constant ; the ordinary 
index ranges from 1766 to 1774 and the extra- 
ordinary index from 1757 to 1765, the double 
refraction remaining always the same, 0*009. 1 he 
amount of colour-dispersion is small, and therefore 



i;6 GEM-STONES 

colourless corundum displays very little ' fire.' The 
difference between the indices for red and blue light 
is, however, sufficiently great that the base of a 
ruby may be left relatively thicker than that of a 
sapphire to secure an equally satisfactory effect 
(cf. p. 98) a point of some importance to the 
lapidary, since stones are sold by weight and it is 
his object to keep the weight as great as possible. 
When a corundum is tested on the refractometer in 
white light a wide spectrum deliminates the two 
portions of the field because of the smallness of the 
colour-dispersion (cf. p. 25). The dichroism of both 
ruby and sapphire is marked, the twin colours given 
by the former being red and purplish-red, and by 
the latter blue and yellowish-blue, the second colour 
in each instance corresponding to the extraordinary 
ray. Tests with the dichroscope easily separate 
ruby and sapphire from any other red or blue stone. 
This character has an important bearing on the 
proper mode of cutting the stones. The ugly 
yellowish tint given by the extraordinary ray of 
sapphire should be avoided by cutting the stone 
with its table-facet at right angles to the prism edge, 
which is the direction of single refraction. Whether 
a ruby should be treated in the same way is a moot 
point. No doubt if the colour is deep, it is the best 
plan, because the amount of absorption of light is 
thereby sensibly reduced, but otherwise the delightful 
nuances distinguishing ruby are best secured by 
cutting the table-facet parallel to the direction of 
single refraction. Yellow corundum also shows 
distinct dichroism, but by a variation more of the 
depth than of the tint of the colour ; the phenomenon 
is faint compared with the dichroic effect of a yellow 



CORUNDUM 177 

chrysoberyl. The specific gravity also is very 
constant, varying only from 3-95 to 4*10; sapphire 
is on the whole lighter than ruby. Corundum has 
the symbol 9 on Mohs's scale, but though coming 
next to diamond it is a very poor second (cf. p. 79). 
As is usually the case, the application of heat tends 
to lighten the colour of the stones : those of a pale 
violet or a yellow colour lose the tint entirely, and 
the deep violet stones turn a lovely rose colour. On 
the other hand the action of radium has, as was 
shown by Bordas, an intensifying action on the 
colour, and even develops it in a colourless stone. 
From the latter reaction it may be inferred that often 
in an apparently colourless stone two or more 
selective influences are at work which ordinarily 
neutralize one another,but, being unequally stimulated 
by the action of radium, they thereupon give rise to 
colour. The stellate appearance of asterias or star- 
stones star-ruby and star-sapphire results from 
the regular arrangement either of numerous small 
channels or of twin-lamellae in the stone parallel to 
the six sides of the prisms ; light is reflected from 
the interior in the form of a six-rayed star (p. 38). 
Some stones from Siam possess a markedly fibrous 
or silky structure. 

The synthetical manufacture of ruby, sapphire, 
and other varieties of corundum has already been 
described (p. 1 1 6). 

Besides its use in jewellery corundum is on ac- 
count of its hardness of great service for many other 
purposes. Small fragments are extensively employed 
for the bearing parts of the movements of watches, 
and both the opaque corundum and the impure 
kind known as emery are in general use for 

12 



1 78 GEM-STONES 

grinding and polishing softer stones, and steel and 
other metal-work. 

The world's supply of fine rubies is drawn almost 
entirely from the famous ruby mines near Mogok, 
situated about 90 miles (145 km.) in a north- 
easterly direction from Mandalay in Upper Burma 
and at a'n elevation of about 4000 ft. (1200 m.) 
above sea-level. It is from this district that the 
stones of the coveted carmine-red, the so-called 
1 pigeon's blood,' colour are obtained. The ruby 
occurs in a granular limestone or calcite in associa- 
tion with the spinel of nearly the same appearance 
the ' balas-ruby,' oriental topaz (yellow cor- 
undum), tourmaline, and occasionally sapphire. 
Some stones are found in the limestone on the 
sides of the hills, but by far the largest quantity 
occur in the k alluvial deposits, both gravel and clay, 
in the river-beds ; the ruby ground is locally 
known as ' byon' The stones are as a rule quite 
small, averaging only about four to the carat. 
Before the British annexation of the country in 
1885 the mines were a monopoly of the Burmese 
sovereigns and were worked solely under royal 
licence. They are known to be of great antiquity, 
but otherwise their early history is a mystery. It 
is said that an astute king secured the priceless 
territory in 1597 from the neighbouring Chinese 
Shans in exchange for a small and unimportant 
town on the Irrawaddy ; if that be so, he struck an 
excellent bargain. The mines were allotted to 
licensed miners, twin-tsas (eaters of the mine) as 
they were called in the language of the country, 
who not only paid for the privilege, but were 
compelled to hand over to the king all stones 



CORUNDUM 179 

above a certain weight As might be anticipated 
this injunction caused considerable trouble, and 
the royal monopolists constantly suspected the miners 
of evading the regulation by breaking up stones 
of exceptional size; from subsequent experience, 
it is probable that large stones were in reality 
seldom found. Since 1887 the mines have been 
worked by arrangement with the Government of 
India by the Ruby Mines, Ltd., an English 
company. Its career has been far from prosper- 
ous, but during recent years, in consequence of 
the improved methods of working the mines and 
of the more generous terms afterwards accorded 
by the Government, greater success has been 
experienced ; the future is, however, to some extent 
clouded by the advent of the synthetical stone, which 
has even made its way out to the East. 

Large rubies are far from common, and such 
as were discovered in the old days were jealously 
hoarded by the Burmese sovereigns. According 
to Streeter the finest that ever came to Europe were 
a pair brought over in 1875, at a time when the 
Burmese king was pressed for money. One, rich 
in colour, was originally cushion-shaped and weighed 
37 carats; the other was a blunt drop in form 
and weighed 47 carats. Both were cut in London, 
the former being reduced to 32^ carats and the 
latter to 38 A carats, and were sold for 10,000 
and 20,000 respectively. A colossal stone, 
weighing 400 carats, is reported to have been found 
in Burma ; it was broken into three pieces, of 
which two were cut and resulted in stones weighing 
70 and 45 carats respectively, and the third was 
sold uncut in Calcutta for 7 lakhs of rupee? 



i8o GEM-STONES 

(46,667). The finder of another large stone 
broke it into two parts, which after cutting weighed 
98 and 74 carats respectively ; he attemped in 
vain to evade the royal acquisitiveness, by giving 
up the larger stone to the king and concealing 
the other. A fine stone, known by the formidable 
appellation of ' Gnaga Boh ' (Dragon Lord), 
weighed 44 carats in the rough and 20 carats 
after cutting. Since the mines were taken over 
by the Ruby Mines, Ltd., a few large stones have 
been discovered. A beautiful ruby was found in 
the Tagoungnandaing Valley, and weighed i8| 
carats in the rough and 1 1 carats after cutting ; 
perfectly clear and of splendid colour, it was sold for 
7000, but is now valued at 10,000. Another, 
weighing 77 carats in the rough, was found in 
1899, and was sold in India in 1904 for 4 lakhs 
of rupees (26,667). A stone, weighing 49 carats, 
was discovered in 1887, and an enormous one, 
weighing as much as 304 carats, in 1890. 

The ruby, as large as a pigeon's egg, which is 
amongst the Russian regalia was presented in 
1777 to the Czarina Catherine by Gustav III of 
Sweden when on a visit to St. Petersburg. The 
large red stone in the English regalia which was 
supposed to be a ruby is a spinel (cf. p. 206). 

Comparatively uncommon as sapphires are in 
the Burma mines a faultless stone, weighing as 
much as 79^ carats, has been discovered there. 

Good rubies, mostly darker in colour than the 
Burmese stones, are found in considerable quantity 
near Bangkok in Siam, Chantabun being the centre 
of the trade, where, just as in Burma, they are 
intimately associated with the red spinel. Because 



CORUNDUM 1 8 1 

of the difference in tint and the consequent 
difference in price, jewellers draw a distinction 
between Burma and Siam rubies ; but that, of 
course, does not signify any specific difference 
between them. Siam is, however, most distin- 
guished as the original home of splendid sapphires. 
Th district of Bo Pie Rin in Battambang produces, 
indeed, more than half the world's supply of 
sapphires. In the Hills of Precious Stones, such 
being the meaning of the native name for the locality, 
a number of green corundums are found. Siam 
also produces brown stones characterized by a 
peculiar silkiness of structure. Rubies are found 
in Afghanistan at the Amir's mines near Kabul 
and also to the north of the lapis lazuli mines 
in Badakshan. 

The conditions in Ceylon are precisely the 
converse of those obtaining in Burma; sapphire is 
plentiful and ruby rare in the island. They are 
found in different rocks, sapphire occurring with 
garnet in gneiss, and ruby accompanying spinel in 
limestone, but they come together in the resulting 
gravels, the principal locality being the gem-district 
near Ratnapura in the south of the island. The 
largest uncut ruby discovered in Ceylon weighed 
42^ carats; it had, however, a decided tinge of blue 
in it. Ceylon is also noted for the magnificent 
yellow corundum, ' oriental topaz,' or, as it is 
locally called, ' king topaz,' which it produces. 

Beautiful sapphires occur in various parts of 
India, but particularly in the Zanskar range of the 
north-western Himalayas in the state of Kashmir, 
where they are associated with brown tourmaline. 
Probably most of the large sapphires known have 



1 82 GEM-STONES 

emanated from India. By far the most gigantic 
ever reported is one, weighing 951 carats, said to 
have been seen in 1827 in the treasury of the 
King of Ava. The collection at the Jardin des 
Plantes contains two splendid rough specimens ; 
one, known as the ' Rospoli,' is quite flawless 
and weighs I32 T V carats, and the other is 2 inches 
in length and i^ inches in thickness. The Duke 
of Devonshire possesses a fine cut stone, weighing 
100 carats, which is brilliant-cut above and step- 
cut below the girdle. An image of Buddha, which 
is cut out of a single sapphire, is exhibited, 
mounted on a gold pin, in the Mineral Gallery of 
the British Museum (Natural History). 

For some years past a large quantity of sap- 
phires have come into the market from Montana, 
U.S.A., especially from the gem-district about 
twelve miles west of Helena. The commonest 
colour is a bluish green, generally pale, but blue, 
green, yellow and occasionally red stones are also 
found ; they are characterized by their almost met- 
allic lustre. With them are associated gold, colour- 
less topaz, kyanite, and a beautiful red garnet which 
is found in grains and usually mistaken for ruby. 
Rubies are also found in limestone at Cowee Creek, 
North Carolina. 

Blue and red corundum, of rather poor quality, 
has come from the Sanarka River, near Troitsk, 
and from Miask, in the Government of Orenburg, 
Russia, and similar stones have been known at 
Campolongo, St. Gothard, Switzerland. 

The prolific gem-district near Anakie, Queens- 
land, supplies examples of every known variety 
of corundum except ruby ; blue, green, yellow, 



CORUNDUM 183 

and parti-coloured stones, and also star-stones, 
are plentiful. Leaf-green corundum is known 
farther south, in Victoria. The Australian sapphire 
is too dark to be of much value. 

Small rubies and sapphires are found in the 
gem-gravels near the Somabula Forest, Rhodesia. 



CHAPTER XX 
BERYL 

(Emerald, Aquamarine, Morganite) 

THE species to be considered in this chapter 
includes the varieties emerald and aquamarine, 
as well as what jewellers understand by beryl. It 
has many incontestable claims on the attention of 
all lovers of the beautiful in precious stones. The 
peerless emerald (Plate I, Fig. 5), which in its ver- 
dant beauty recalls the exquisite lawns that grace 
the courts and quadrangles of our older seats of 
learning, ranks to-day as the most costly of jewels. 
Its sister stone, the lovely aquamarine (Plate I, 
Fig. 4), which seems to have come direct from some 
mermaid's treasure-house in the depths of a summer 
sea, has charms not to be denied. Pliny, speaking 
of this species, truly says, " There is not a colour 
more pleasing to the eye " ; yet he knew only the 
comparatively inferior stones from Egypt, and 
possibly from the Ural Mountains. Emeralds are 
favourite ring-stones, and would, no doubt, be equally 
coveted for larger articles of jewellery did not the 
excessive cost forbid, and nothing could be more 
attractive for a central stone than a choice aqua- 
marine of deep blue- green hue. Emeralds are 

usually step-cut, though Indian lapidaries often 
184 



BERYL 185 

favour the en cabochon form ; aquamarines, on the 
other hand, are brilliant-cut in front and step-cut at 
the back. 

Beryl, to use the name by which the species is 
known to science, is essentially a silicate of aluminium 
and beryllium corresponding to the formula, Be 3 Al 2 
(SiO 3 ) 6 . The beryllia is often partially replaced by 
small amounts of the alkaline earths, caesia, potash, 
soda, and lithia, varying from about i| per cent, in 
beryl from Mesa Grande to nearly 5 in that from 
Pala and Madagascar, and over 6, of which 3-6 is 
caesia, in beryl from Hebron, Maine ; also, as usual, 
chromic and ferric oxides take the place of a little 
alumina ; from I to 2 per cent, of water has been 
found in emerald. The element beryllium was, as 
its name suggests, first discovered in a specimen of 
this species, the discovery being made in 1798 by 
the chemist Vauquelin ; it is also known as glucinum 
in allusion to the sweet taste of its salts. 

When pure, beryl is colourless, but it is rarely, if 
ever, free from a tinge of blue or green. The colour 
is usually some shade of green grass-green, of that 
characteristic tint which is in consequence known as 
emerald-green, or blue-green, yellowish green (Plate 
I, Fig. 6), and sometimes yellow, pink, and rose- 
red. The peculiar colour of emerald is supposed to 
be caused by chromic oxide, small quantities of 
which have been detected in it by chemical analysis ; 
moreover, experiment shows that glass containing 
the same percentage amount of chromic oxide 
assumes the same splendid hue. Emerald, on being 
heated, loses water, but retains its colour unimpaired, 
which cannot therefore be due, as has been suggested, 
to organic matter. The term aquamarine is applied 



1 86 GEM-STONES 

to the deep sea-green and blue-green stones, and 
jewellers restrict the term beryl to paler shades and 
generally other colours, such as yellow, golden, and 
pink, but Kunz has recently proposed the name 
morganite to distinguish the beautiful rose beryl such 
as is found in Madagascar. The varying shades of 
aquamarine are due to the influence of the alkaline 
earths modified by the presence of ferric oxide or 
chromic oxide ; the beautiful blushing hue of mor- 
ganite is no doubt caused by lithia. 

The name of the species is derived from the Greek 
/3?7/3iA\os, an ancient word, the meaning of which 
has been lost in the mists of time. 
The Greek word denoted the same 
species in part as that now under- 
stood by the name. Emerald is 
derived from a Persian word which 
appeared in Greek as (r/jbdpaySos, and 
in Latin as smaragdus\ it originally 
denoted chrysocolla, or similar green 
stone, but was transferred upon the introduction of 
the deep-green beryl from Upper Egypt. The name 
aquamarine was suggested by Pliny's exceedingly 
happy description of the stones " which imitate the 
greenness of the clear sea," although it was not actu- 
ally used by him. That emerald and beryl were one 
species was suspected by Pliny, but the identity was 
not definitely established till about a century ago. 
Morganite is named after John Pierpont Morgan. 

The natural crystals have the form of a six-sided 
prism, and in the case of emerald (Fig. 70, and 
Plate I, Fig. 8) invariably, if whole, end in a 
single face at right angles to the length of the 
prism ; aquamarines have in addition a number of 




BERYL 187 

small inclined faces, and stones from both Russia 
and Brazil often taper owing to the effects of 
corrosion. The sixfold character of the crystalline 
symmetry necessarily entails that the double 
refraction, which is small in amount, 0*006, 
is uniaxial in character, and, since the ordinary 
is greater than the extraordinary refractive 
index, it is negative in sign. The values of 
the indices range between 1*567 and 1*590, and 
1-572 and I -598 respectively, in the two cases, the 
pink beryl possessing the highest values. The 
dichroism is distinct in the South American emerald, 
the twin colours being yellowish and bluish green, 
but otherwise is rather faint. The specific gravity 
varies between 2*69 and 279, and is therefore a 
little higher than that of quartz. If, therefore, a 
beryl and a quartz be floating together in a tube 
containing a suitable heavy liquid, the former will 
always be at a sensibly lower level (cf. Fig. 32). 
The hardness varies from 7| to 8, emerald being a 
little softer than the other varieties. There is no 
cleavage, but like most gem-stones beryl is very 
brittle, and can easily be fractured. Stones rendered 
cloudy by fissures are termed ' mossy.' When 
heated before the blowpipe beryl is fusible with 
difficulty ; it resists the attack of hydrofluoric acid 
as well as of ordinary acids. 

In all probability the whole of the emeralds 
known in ancient times came from the so-called 
Cleopatra emerald mines in Upper Egypt. For 
some reason they were abandoned, and their position 
was so completely lost that in the Middle Ages it 
was maintained that emeralds had never been found 
in Egypt at all, but had come from America by way 



1 88 GEM-STONES 

of the East. All doubts were set at rest by the 
re-discovery of the mines early last century by 
Cailliaud, who had been sent by the Viceroy of 
Egypt to search for them. They were, however, 
not much worked, and after a few years were closed 
again, and were re-opened only about ten years ago. 
The principal mines are at Jebel Zabara and at 
Jebel Sikait in northern Etbai, about 10 miles 
(16 km.) apart and distant about 15 miles (24 km.) 
from the Red Sea, lying in the range of mountains 
that run for a long distance parallel to the west 
coast of the Red Sea and rise to over 1800 feet 
(550 m.) above sea-level. There are numerous signs 
of considerable, but primitive, workings at distinct 
periods. Both emeralds and beryls are found in 
micaceous and talcose schists. The emeralds are 
not of very good quality, being cloudy and rather 
light in colour. Finer emeralds have been found in 
a dark mica-schist, together with other beryllium 
minerals, chrysoberyl and phenakite, and also topaz 
and tourmaline on the Asiatic side of the Ural 
Mountains, near the Takowaja River, which flows into 
the Bolshoi Reft River, one of the larger tributaries of 
the Pyschma River, about fifty miles (80 km.) east of 
Ekaterinburg, a town which is chiefly concerned with 
the mining and cutting of gem-stones. The mine 
was accidentally discovered by a peasant, who noticed 
a few green stones at the foot of an uprooted tree in 
1830. Two years later the mine was regularly 
worked, and remained open for twenty years, when 
it was closed. It has recently been re-opened 
owing to the high rates obtaining for emeralds. 
Very large crystals have been produced here, but in 
colour they are much inferior to the South American 



BERYL 189 

stones ; small Siberian emeralds, on the other hand, 
are of better colour than small South American 
emeralds, the latter being not so deep in tint. 
Emeralds have been found in a similar kind of schist 
at Habachtal, in the Salzburg Alps. About thirty 
years ago well-formed green stones were discovered 
with hiddenite at Stony Point, Alexander County, 
in North Carolina, but not much gem material has 
come to light 

The products of none of the mines that have just 
been mentioned can on the whole compare with the 
beautiful stones which have come from South 
America. At the time when the Spaniards grimly 
conquered Peru and ruthlessly despoiled the country 
of the treasures which could be carried away, 
immense numbers of emeralds some of almost 
incredible size were literally poured into Spain, 
and eventually found their way to other parts of 
Europe. These stones were known as Spanish or 
Peruvian emeralds, but in all probability none of 
them were actually mined in Peru. Perhaps the 
most extraordinary were the five choice stones which 
Cortez presented to his bride, the niece of the Duke 
de Bejar, thereby mortally offending the Queen, who 
had desired them for herself, and which were lost in 
i 529 when Cortez was shipwrecked on his disastrous 
voyage to assist Charles V at the siege of Algiers. 
All five stones had been worked to divers fantastic 
shapes. One was cut like a bell with a fine pearl 
for a tongue, and bore on the rim, in Spanish, 
" Blessed is he who created thee." A second was 
shaped like a rose, and a third like a horn. A 
fourth was fashioned like a fish, with eyes of gold. 
The fifth, which was the most valuable and the most 



190 GEM-STONES 

remarkable of all, was hollowed out into the form of 
a cup, and had a foot of gold ; its rim, which was 
formed of the same precious metal, was engraved 
with the words, " Inter natos mulierum non surrexit 
major." As soon as the Spaniards had seized nearly 
all the emeralds that the natives had amassed in 
their temples or for personal adornment, they de- 
voted their attention to searching for the source of 
these marvels of nature, and eventually in 1558 
they lighted by accident upon the mines in what is 
now the United States of Colombia, which have been 
worked almost continuously since that time. Since 
the natives, who naturally resented the gross injustice 
with which they had been treated, and penetrated 
the greed that prompted the actions of the Spaniards, 
hid all traces of the mines, and refused to give any 
information as to their position, it is possible that 
other emerald mines may yet be found. The 
present mines are situated near the village of Muzo, 
about 75 miles (120 km.) north-north-west of 
Bogota, the capital of Colombia. The emeralds 
occur in calcite veins in a bituminous limestone of 
Cretaceous age. The Spaniards formerly worked 
the mines by driving adits through the barren rock 
on the hillsides to the gem-bearing veins, but at the 
present day the open cut method of working is 
employed. A plentiful supply of water is available, 
which is accumulated in reservoirs and allowed at 
the proper time to sweep the debris of barren rock 
away into the Rio Minero, leaving the rock contain- 
ing the emeralds exposed. Stones, of good quality, 
which are suited for cutting, are locally known as 
canutillos, inferior stones, coarse or ill-shaped, being 
called morallons. 



BERYL 191 

Emerald, unlike some green stones, retains its 
purity of colour in artificial light ; in fact, to quote 
the words of Pliny, " For neither sun nor shade, nor 
yet the light of candle, causeth to change and lose 
their lustre." Many are the superstitions that have 
been attached to it. Thus it was supposed to be 
good for the eyes, and as Pliny says, " Besides, there 
is not a gem or precious stone that so fully 
possesseth the eye, and yet never contenteth it with 
satiety. Nay, if the sight hath been wearied and 
dimmed by intentive poring upon anything else, the 
beholding of this stone doth refresh and restore it 
again." The idea that it was fatal to the eyesight 
of serpents appears in Moore's lines 

"Blinded like serpents when they gaze 
Upon the emerald's virgin blaze." 

The crystals occur attached to the limestone, and 
are therefore never found doubly terminated. The 
crystal form is very simple, merely a hexagonal 
prism with a flat face at the one end at right angles 
to it. They are invariably flawed, so much so that 
a flawless emerald has passed into proverb as un- 
attainable perfection. The largest single crystal 
which is known to exist at the present day is in the 
possession of the Duke of Devonshire (Fig. 71). In 
section it is nearly a regular hexagon, about 2 inches 
(5 1 mm.) in diameter from side to side, and the 
length is about the same; its weight is 27679 
grams (9f oz. Av., or 1347 carats). It is of good 
colour, but badly flawed. It was given to the Duke 
of Devonshire by Dom Pedro of Brazil, and was 
exhibited at the Great Exhibition of 1851. A 
fine, though much smaller crystal, but of even better 



192 



GEM-STONES 



colour, which weighs 32*2 grams (156^- carats), 
and measures i-|. inch (28 mm.) in its widest cross- 
diameter, and about the same in length, was acquired 
with the Allan- Greg collection by the British 
Museum, and is exhibited in the Mineral Gallery 




FIG. 71. Duke of Devonshire's Emerald. 
(Natural size.) 

of the British Museum (Natural History). The 
finest cut emerald is said to be one weighing 30 
carats, which belongs to the Czar of Russia. A 
small, but perfect and flawless, faceted emerald, 
which is set in a gold hoop, is also in the British 
Museum (Natural History). It is shown, without 
the setting, about actual size, on Plate I, Fig. 5. 



BERYL 193 

The ever great demand and the essentially re- 
stricted supply have forced the cost of emeralds of 
good quality to a height that puts large stones 
beyond the reach of all but a privileged few who 
have purses deep enough. The rate per carat may 
be anything from 15 upwards, depending upon the 
purity of the colour and the freedom from flaws, but 
it increases very rapidly with the size, since flawless 
stones of more than 4 carats or so in weight are 
among the rarest of jewels ; a perfect emerald of 4 
carats may easily fetch 1600 to ^2000. It seems 
anomalous to say that it has never been easier to 
procure fine stones than during recent years, but 
the reason is that the high prices prevailing have 
tempted owners of old jewellery to realize their 
emeralds. On the other hand, pale emeralds are 
worth only a nominal sum. 

The other varieties of beryl are much less rare, 
and, since they usually attain to more considerable, 
and sometimes even colossal, size, far larger stones 
are obtainable. An aquamarine, particularly of good 
deep blue-green colour, is a stone of great beauty, 
and it possesses the merit of preserving its purity 
of tint in artificial light. It is a favourite stone for 
pendants, brooches, and bracelets, and all purposes 
for which a large blue or green stone is desired. 
The varying tints are said to be due to the presence 
of iron in different percentages, and possibly in 
different states of oxidation. Unlike emerald, the 
other varieties are by no means so easily recognized 
by their colour. Blue aquamarines may easily be 
mistaken for topaz, or vice versa, and the yellow 
beryl closely resembles other yellow stones, such as 
quartz, topaz, or tourmaline. Stones which are 
13 



194 GEM-STONES 

colourless or only slightly tinted command little 
more than the price of cutting, but the price of 
blue-green stones rapidly advances with increasing 
depth of tint up to 2 a carat: The enormous 
cut aquamarine which is exhibited in the Mineral 
Gallery of the British Museum (Natural History), 
affords some idea of the great size such stones reach ; 
a beautiful sea-green in colour, it weighs 179*5 
grams (875 carats), and is table-cut with an oval 
contour. 

The splendid six-sided columns which have been 
discovered in various parts of Siberia are among the 
most striking specimens in any large mineral collec- 
tion. The neighbourhood of Ekaterinburg in the 
Urals is prolific in varieties of aquamarine ; especially 
at Mursinka have fine stones been found, in associa- 
tion with topaz, amethyst, and schorl, the black 
tourmaline. Good stones also occur in conjunction 
with topaz at Miask in the Government of Orenburg. 
It is found in the gold-washings of the Sanarka 
River, in the Southern Urals, but the stones are not 
fitted for service as gems. Magnificent blue-green 
and yellow aquamarines are associated with topaz 
and smoky quartz in the granite of the Adun- 
Tschilon Mountains, near Nertschinsk, Transbaikal. 
Stones have also been found at the Urulga River in 
Siberia. Most of the bluish-green aquamarines 
which come into the market at the present time 
have originated in Brazil, particularly in Minas 
Novas, Minas Geraes, where clear, transparent stones, 
of pleasing colour, in various shades, are found in 
the utmost profusion ; beautiful yellow stones also 
occur at the Bahia mines. Aquamarine was 
obtained in very early times in Coimbatore District, 



BERYL 195 

Madras, India, and yellow beryl comes from Ceylon. 
Fine blue crystals occur in the granite of the Mourne 
Mountains, Ireland, but they are not clear enough 
for cutting purposes ; similar stones are found also 
at Limoges, Haute Vienne, France. Aquamarines 
of various hues abound in several places in the 
United States, among the principal localities being 
Stoneham in Maine, Haddam in Connecticut, and 
Pala and Mesa Grande in San Diego County, 
California. The last-named state is remarkable for 
the numerous stones of varying depth of salmon- 
pink that have been found there. It is, however, 
surpassed by Madagascar, which has recently pro- 
duced splendid stones of perfect rose-red tint and 
of the finest gem quality, some of them being 
nearly 100 carats in weight. These stones, which 
have been assigned a special name, morganite (cf. 
supra), are associated with tourmaline and kunzite. 
Pink and yellow beryls and deep blue-green aqua- 
marines occur in the island in quantity. The pink 
beryls from California are generally pale or have a 
pronounced salmon tint, and seldom approach the 
real rose-red colour of morganite ; one magnificent 
rose-red crystal, weighing nearly 9 Ib. (4*05 kg.), 
has, however, been recently discovered in San Diego 
County, California, and is now in the British 
Museum (Natural History). Blue-green beryl, 
varying in tint from almost colourless to an 
emerald-green, occurs with tin-stone and topaz 
about 9 miles (14^ km.) north-east of Emmaville in 
New South Wales, Australia. 

Probably the largest and finest aquamarine crystal 
ever seen was one found by a miner on March 28, 
1910, at a depth of 15 ft. (5 m.) in a pegmatite vein 



196 GEM-STONES 

at Marambaya, near Arassuahy, on the Jequitinhonha 
River, Minas Geraes, Brazil. It was greenish blue 
in colour, and a slightly irregular hexagonal prism, 
with a flat face at each end, in form ; it measured 
19 in. (48'5 cm.) in length and 16 in. (41 cm.) in 
diameter, and weighed 243 Ib. (iio - 5 kg.); and its 
transparency was so perfect that it could be seen 
through from end to end (Plate XXVI). The 
crystal was transported to Bahia, and sold for 
$25,000 (5133)- 



PLATE XXVI 




RGE AQUAMARINE CRYSTAL (one-sixth natural size), FOUND AT 
MINAS GERAES, BRAZIL 



PART II SECTION B 
SEMI-PRECIOUS STONES 

CHAPTER XXI 
TOPAZ 

TOPAZ is the most popular yellow stone in 
jewellery, and often forms the principal 
stone in brooches or pendants, especially in old- 
fashioned articles. It is a general idea that all 
yellow stones are topazes, and all topazes are 
yellow ; but neither statement is correct. A very 
large number of yellow stones that masquerade as 
topaz are really the yellow quartz known as citrine. 
The latter is, indeed, almost universally called by 
jewellers topaz, the qualification ' Brazilian ' being 
used by them to distinguish the true topaz. Many 
species besides those mentioned yield yellow stones. 
Thus corundum includes the beautiful ' oriental 
topaz' or yellow sapphire, and yellow tourmalines 
are occasionally met with ; the yellow chrysoberyl 
always has a greenish tinge. Topaz is generally 
brilliant-cut in front and step-cut at the back, and 
the table facet is sometimes rounded, but the 
colourless stones are often cut as small brilliants; 
it takes an excellent and dazzling polish. 



198 GEM-STONES 

Topaz is a silicate of aluminium corresponding 
to the formula [Al(F,OH)] 2 SiO 4 , which was estab- 
lished in 1894 by Penfield and Minor as the result 
of careful research. Contrary to the general idea, 
topaz is usually colourless or very pale in tint. 
Yellow hues of different degrees, from pale to a 
rich sherry tint (Plate I, Fig. 9), are common, 
and pure pale blue (Plate I, Fig. 7) and pale green 
stones, which often pass as aquamarine, are far 
from rare. Natural, red and pink, stones are very 
seldom to be met with. It is, however, a peculiarity 
of the brownish-yellow stones from Brazil that the 
colour is altered by heating to a lovely rose-pink. 
Curiously, the tint is not apparent when the stone 
is hot, but develops as it cools -to a normal tempera- 
ture ; the colour seems to be permanent. Such 
stones are common in modern jewellery. Although 
the change in colour is accompanied by some slight 
rearrangement of the constituent molecules, since 
such stones are invariably characterized by high 
refraction and pronounced dichroism, the crystalline 
symmetry, however, remaining unaltered, the cause 
must be attributed to some change in the tinctorial 
agent, probably oxidation. The yellow stones from 
Ceylon, if treated in a similar manner, lose their 
colour entirely. The pale yellow-brown stones from 
Russia fade on prolonged exposure to strong sun- 
light, for which reason the superb suite of crystals 
from the Urulga River, which came with the 
Koksharov collection to the British Museum, are 
kept under cover. 

The name of the species is derived from topazion 
(T07raen>, to seek), the name given to an island in 
the Red Sea, which in olden times was with difficulty 



TOPAZ 



199 



located, but it was applied by Pliny and his con- 
temporaries to the yellowish peridot found there. 
The term was applied in the Middle Ages loosely 
to any yellow stone, and was gradually applied 
more particularly to the stone that was then more 
prevalent, the topaz of modern science. As has 
already been pointed out (p. ill), the term is still 
employed in jewellery to signify any yellow stone. 
The true topaz was probably included by Pliny 
under the name chrysolithus. 

The symmetry is orthorhombic, and the crystals 
are prismatic in shape and ter- 
minated by numerous inclined 
faces, and usually by a large face 
perpendicular to the prism edge 
(Fig. 72). Topaz cleaves with 
great readiness at right angles 
to the prism edge ; owing to its 
facile cleavage, flaws are easily 
started, and caution must be 
exercised not to damage a stone 
by knocking it against hard and unyielding sub- 
stances. The dichroism of a yellow topaz is 
always perceptible, one of the twin colours being 
distinctly more reddish than the other, and the 
phenomenon is very marked in the case of stones 
the colour of which has been artificially altered to 
pink. The values of the least and the greatest of 
the principal indices of refraction vary from 1*615 
to 1-629, and from 1-625 to 1-637, respectively, 
the double refraction being about 0*010 in amount, 
and positive in sign. The high values correspond 
to the altered stones. The specific gravity, the 
mean value of which is 3-55 with a variation of 



\ 



FIG. 72. Topaz 
Crystal. 



200 GEM-STONES 

0*05 on either side, is higher than would be ex- 
pected from the refractivity. A cleavage flake 
exhibits in convergent polarized light a wide- 
angled biaxial picture, the ' eyes ' lying outside 
the field of view. The relation of the principal 
optical directions and the directions of single re- 
fraction to the crystal are shown in Fig. 27. The 
hardness is 8 on Mohs's scale, and in this character 
it is surpassed only by chrysoberyl, corundum, 
and diamond. Topaz is pyro-electric, in which 
respect tourmaline alone exceeds it, and it may be 
strongly electrified by friction. 

Although the range of refraction overlaps that 
of tourmaline, there is no risk of confusion, because 
the latter has nearly thrice the amount of double 
refraction (cf. p. 29). Apart from the difference 
in refraction, a yellow topaz ought never to be 
confused with a yellow quartz, because the former 
sinks, and the latter floats in methylene iodide. 
The same test distinguishes topaz from beryl, and, 
indeed, from tourmaline also. 

Judged by the criterion of price, topaz is not in 
the first rank of precious stones. Stones of good 
colour and free from flaws are now, however, scarce. 
Pale stones are worth very little, possibly less than 
43. a carat, but the price rapidly advances with 
increase in colour, reaching 2os. for yellow, 8os. 
for pink and blue stones. Since topazes are pro- 
curable in all sizes customary in jewellery, the rates 
vary but slightly, if at all, with the size. 

Topaz occurs principally in pegmatite dykes and 
in cavities in granite, and is interesting to petrolo- 
gists as a conspicuous instance of the result of the 
action of hot acid vapours upon rocks rich in 



TOPAZ 201 

aluminium silicates. Magnificent crystals have 
come from the extensive mining district which 
stretches along the eastern flank of the Ural 
Mountains, and from the important mining region 
surrounding Nertschinsk, in the Government of 
Transbaikal, Siberia. Fine green and blue stones 
have been found at Alabashka, near Ekaterinburg, 
in the Government of Perm, and at Miask in the 
Ilmen Mountains, in the Government of Orenburg. 
Topazes of the rare reddish hue have been picked 
out from the gold washings of the Sanarka River, 
Troisk, also in the Government of Orenburg. 
Splendid pale-brown stones have issued from the 
Urulga River, near Nertschinsk, and good crystals 
have come from the Adun-Tschilon Mountains. 
Kamchatka has produced yellow, blue, and green 
stones. In the British Isles, beautiful sky-blue, 
waterworn crystals have been found at Cairngorm, 
Banffshire, in Scotland, and colourless stones in 
the Mourne Mountains, Ireland, and at St. Michael's 
Mount, Cornwall. Most of the topazes used in 
jewellery of the present day come from either 
Brazil or Ceylon. Ouro Preto, Villa Rica, and 
Minas Novas, in the State of Minas Geraes, are 
the principal localities in Brazil. Numerous stones, 
often waterworn, brilliant and colourless or tinted 
lovely shades of blue and wine-yellow, occur there ; 
reddish stones also have been found at Ouro Preto. 
Ceylon furnishes a profusion of yellow, light-green, 
and colourless, waterworn pebbles. The colourless 
stones found there are incorrectly termed by the 
natives ' water-sapphire,' and the light-green stones 
are sold with beryl as aquamarines ; the stones 
locally known as ' king topaz ' are really yellow 



202 GEM-STONES 

corundum (cf. p. 181). Colourless crystals, some- 
times with a faint tinge of colour, have been dis- 
covered in many parts of the world, such as 
Ramona, San Diego County, California, and Pike's 
Peak, Colorado, in the United States, San Luis 
Potosi in Mexico, and Omi and Otami-yama in 
Japan. 



CHAPTER XXII 
SPINEL 

(Balas-Ruby, Rubicelle) 

SPINEL labours under the serious disadvantage 
of being overshadowed at almost all points 
by its opulent and more famous cousins, sapphire 
and ruby, and is not so well known as it deserves 
to be. The only variety which is valued as a 
gem is the rose-tinted stone called balas-ruby (Plate 
XXVII, Fig. 3), which is very similar to the true ruby 
in appearance; they are probably often confused, 
especially since they are found in intimate associa- 
tion in nature. Spinels of other colours are not 
very attractive to the eye, and are not likely to be 
in much demand. Blue spinel (Plate XXVII, Fig. 4) 
is far from common, but the shade is inclined to 
steely-blue, and is much inferior to the superb tint 
of the true sapphire. Spinel is very hard and 
eminently suitable for a ring-stone, but is seldom 
large and transparent enough for larger articles 
of jewellery. 

Spinel is an aluminate of magnesium corre- 
sponding to the formula MgAl 2 O 4 , and therefore is 
closely akin to corundum, alumina, and chrysoberyl, 
aluminate of beryllium. The composition may, 
however, vary considerably owing to the isomor- 
303 



204 GEM-STONES 

phous replacement of one element by another ; in 
particular, ferrous oxide or manganese oxide often 
takes the place of some magnesia, and ferric oxide 
or chromic oxide is found instead of part of the 
alumina. When pure, spinel is devoid of colour, 
but such stones are exceedingly rare. No doubt 
chromic oxide is responsible for the rose-red hue 
of balas-ruby, and also, when tempered by ferric 
oxide, for the orange tint of rubicelle, and man- 
ganese is probably the cause of the peculiar violet 
colour of almandine-spinel. It is scarcely possible 
to define all the shades between blue and red that 
may be assumed by spinel. Stones which are rich 
in iron are known as pleonaste or ceylonite ; they 
are quite opaque, but are sometimes used for orna- 
mental wear. 

The name of the species comes from a diminutive 
form of O-TTIVOS, a spark, and refers to the fiery red 
colour of the most valued kind of spinel. It may 
be noted that the Latin equivalent of the word, 
carbunculus, has been applied to the crimson garnet 
when cut en cabochon. Balas is derived from 
Balascia, the old name for Badakshan, the district 
from which the finest stones were brought in 
mediaeval times. 

Spinel, like diamond, belongs to the cubic system 
of crystalline symmetry, and occurs in beautiful octa- 
hedra, or in flat triangular-shaped plates (Figs. 73, 74) 
the girdles of which are cleft at each corner, these 
plates being really twinned octahedra. The refrac- 
tion is, of course, single, and there is therefore no 
double refraction or dichroism ; this test furnishes 
the simplest way of discriminating between the 
balas and the true ruby. Owing to isomorphous 



SPINEL 205 

replacement the value of the refractive index may 
lie anywhere between 1716 and 1736. The lower 
values, about 1720, correspond to the most trans- 
parent red and blue stones ; the deep violet stones 
have values above 1730. Spinel possesses little 
colour-dispersion, or ' fire.' In the same way the 
values of the specific gravity, even of the trans- 
parent stones, vary between 3-5 and 37, but the 
opaque ceylonite has values as high as 4*1. Spinel 
is slightly softer than sapphire and ruby, and has 
the symbol 8 on Mohs's scale, and it is scarcely 
inferior in lustre 
to these stones. 
Spinel is easily 
separated from 
garnet of similar 
colour by its 
lower refractivity. 
Spinels run from FlGS . 73j 74 ._s p inel Crystals, 

i os. to 5 a carat, 

depending on their colour and quality, and excep- 
tional stones command a higher rate. 

Spinel always occurs in close association with 
corundum. The balas and the true ruby are mixed 
together in the limestones of Burma and Siam. 
Curiously enough, the spinel despite its lower hard- 
ness is found in the river gravels in perfect crystals, 
whereas the rubies are generally waterworn. Fine 
violet and blue spinels occur in the prolific gem- 
gravels of Ceylon. A large waterworn octahedron 
and a rough mass, both of a fine red colour, are 
exhibited in the Mineral Gallery of the British 
Museum (Natural History), and a beautiful faceted 
blue stone is shown close by. 




206 GEM-STONES 

The enormous red stone, oval in shape, which is 
set in front of the English crown, is not a ruby, as 
it was formerly believed to be, but a spinel. It was 
given to the gallant Black Prince by Pedro the 
Cruel after the battle of Najera in 1367, and was 
subsequently worn by Henry V upon his helmet at 
the battle of Agincourt. As usual with Indian- 
fashioned stones it is pierced through the middle, 
but the hole is now hidden by a small stone of 
similar colour. 

The British Regalia also contains the famous stone 
called the Timur Ruby or Khiraj-i-Alam (Tribute of 
the World), which weighs just over 352 carats, and 
is the largest spinel-ruby known. It is uncut, but 
polished. Its history goes back to 1398, when it 
was captured by the Amir Timur at Delhi. On the 
wane of the Tartar empire the stone became the pro- 
perty of the Shahs of Persia, until it was given by 
Abbas I to his friend and ally, the Mogul Emperor, 
Jehangir. It remained at Delhi until, on the sack of 
that city by Nadir Shah in 1739, it, together with 
immense booty, including the Koh-i-nor, fell into the 
hands of the conqueror. Like the great diamond, it 
eventually came into the possession of Runjit Singh 
at Lahore, and on the annexation of the Punjab in 
1850 passed to the East India Company. It was 
shown at the Great Exhibition of 1851, and after- 
wards presented to Queen Victoria. 

Mention has been made above (p. 121) of the 
blue spinel which is manufactured in imitation of 
the true sapphire. The artificial stone is quite 
different in tint from the blue spinel found in 
nature. 



CHAPTER XXIII 
GARNET 

THE important group of minerals which are 
known under the general name of garnet 
provides an apt illustration of the fact that rarity 
is an essential condition if a stone is to be accounted 
precious. Owing to the large quantity of Bohemian 
garnets, of a not very attractive shade of yellowish 
red, that have been literally poured upon the market 
during the past half-century the species has become 
associated with cheap and often ineffective jewellery, 
and has acquired a stigma which completely pre- 
vents its attaining any popularity with those pro- 
fessing a nice taste in gem-stones. It must not, 
however, be supposed that garnet has entirely dis- 
appeared from high-class jewellery although the 
name may not readily be found in a jeweller's 
catalogue. Those whose business it is to sell gem- 
stones are fully alive to the importance of a name, 
and, as has already been remarked (p. 109), they 
have been fain to meet the prejudices of their 
customers by offering garnets under such misleading 
guises as ' Cape-ruby,' ' Uralian emerald,' or ' olivine.' 
Garnets may, moreover, figure under another 
name quite unintentionally. Probably many a 
fine stone masquerades as a true ruby ; the im- 
possibility of distinguishing these two species in 



208 GEM-STONES 

certain cases by eye alone is perhaps not widely 
recognized. An instructive instance came under 
the writer's notice a few years ago. A lady one 
day had the misfortune to fracture one of the stones 
in a ruby ring that had been in the possession of 
her family for upwards of a century, and was origin- 
ally purchased of a leading firm of jewellers in 
London. She took the ring to her jeweller, and 
asked him to have the stone replaced by another 
ruby. A day or two later he sent word that it 
was scarcely worth while to put a ruby in because 
the stones in the ring were paste. Naturally dis- 
tressed at such an opinion of a ring which had 
always been held in great esteem by her family, 
the lady consulted a friend, who suggested showing 
it to the writer. A glance was sufficient to prove 
that if the ring had been in use so long the stones 
could not possibly be paste on account of the 
excellent state of their polish, but a test with the 
refractometer showed that the stones were really 
almandine-garnets, which so often closely resemble 
the true ruby in appearance. Beautiful as the 
stones were, the ring was probably not worth one- 
tenth what the value would have been had the 
stones been rubies. 

To the student of mineralogy garnet is for many 
reasons of peculiar interest. It affords an excellent 
illustration of the facility which certain elements 
possess for replacing one another without any great 
disturbance of the crystalline form. Despite their 
apparent complexity in composition all garnets con- 
form to the same type of formula : lime, magnesia, 
and ferrous and manganese oxides, and again alumina 
and ferric and chromic oxides may replace each other 



GARNET 209 

in any proportion, iron being present in two states 
of oxidation, and it would be rare to find a stone 
which agrees in composition exactly with any of 
the different varieties of garnet given below. 

Garnet belongs to the cubic system of crystalline 
symmetry. Its crystals are commonly of two kinds, 
both of which are very characteristic, the regular 
dodecahedron, i.e. twelve-faced figure (Fig. 75), and 
the tetrakis-octahedron or three-faced octahedron 
(Fig. 76); the latter crystals are, especially when 
weather- or water- worn, almost spherical in shape. 
Closer and more refined observations have shown 
that garnet is sel- 
dom homogeneous, 
being usually com- 
posed of several 
distinct individuals 
of a lower order 
of symmetry. Al- 
though singly re- 
fractive as far as. can be determined. with the refracto- 
meter or by deviation through a prism, yet when 
examined under the polarizing microscope, garnets 
display invariably a small amount of local double 
refraction. The transition from light to darkness is, 
however, not sharp as in normal cases, but is pro- 
longed into a kind of twilight. In hardness, garnet 
is on the whole about the same as quartz, but varies 
slightly ; hessonite and andradite are a little softer, 
pyrope, spessartite, and almandine are a little harder, 
while uvarovite is almost the same. All the varieties 
except uvarovite are fusible when heated before 
the blowpipe, and small fragments melt sufficiently 
on the surface in the ordinary bunsen flame to 




210 GEM-STONES 

adhere to the platinum wire holding them. This 
test is very useful for separating rough red garnets, 
pyrope or almandine, from red spinels or zircons 
of very similar appearance. Far greater variation 
occurs in the other physical characters. The specific 
gravity may have any value between 3*55 and 
4-20, and the refractive index ranges between 1740 
and I '890. Both the specific gravity and the re- 
fractive index increase on the whole with the per- 
centage amount of iron. 

Garnet is a prominent constitutent of many 
kinds of rocks, but the material most suitable for 
gem purposes occurs chiefly in crystalline schists or 
metamorphic limestones. Pyrope and demantoid are 
furnished by peridotites and the serpentines result- 
ing from them ; almandine and spessartite come 
mostly from granites. 

The name of the species is derived from the 
Latin granatus, seed-like, and is suggested by the 
appearance of the spherical crystals when embedded 
in their pudding-like matrix. 

The varieties most adapted to jewellery are the 
fiery-red pyrope and the crimson and columbine-red 
almandine ; the closer they approach the ruddy hue 
of ruby the better they are appreciated. Hessonite 
was at one time in some demand, but it inclines too 
much to the yellowish shade of red and possesses 
too little perfection of transparency to accord with 
the taste of the present day. Demantoid provides 
beautiful, pale and dark emerald-green stones, of 
brilliant lustre and high dispersion, which are 
admirably adapted for use in pendants or necklaces ; 
on account of their comparative softness it would be 
unwise to risk them in rings. In many stones the 



GARNET 211 

colour takes a yellowish shade, which is less in 
demand. Uvarovite also occurs in attractive 
emerald-green stones, but unfortunately none as yet 
have been found large enough for cutting. A few 
truly magnificent spessartites are known one, a 
splendid example, weighing 6f- carats, being in the 
possession of Sir Arthur Church ; but the species 
is far too seldom transparent to come into general 
use. The price varies per carat from 2s. for 
common garnet to IDS. for stones most akin to 
ruby in colour, and exceptional demantoids may 
realize even as much as 10 a carat. The old style 
of cutting was almost invariably rounded or en 
cabochon, but at the present day the brilliant-cut front 
and the step-cut back is most commonly adopted. 

The several varieties will now be considered in 
detail. 

(a) HESSONITE 

(Grossular, Cinnamon- Stone, Hyacinth, JacintJt) 

This variety, strictly a calcium-aluminium garnet 
corresponding to the formula Ca 3 Al 2 (SiO 4 ) 3 , but 
generally containing some ferric oxide and there- 
fore tending towards andradite, is called by several 
different names. In science it is usually termed 
grossular, a word derived from grossularia, the 
botanical name for gooseberry, in allusion to 
the colour and appearance of many crystals, or 
hessonite, and less correctly essonite, words derived 
from the Greek r\a<rwv in reference to the inferior 
hardness of these stones as compared with zircon 
of similar colour ; in jewellery it is better known 
as cinnamon-stone, if a golden-yellow in colour, or 
hyacinth or jacinth. The last word, which is in- 



212 GEM-STONES 

discriminately used for hessonite and yellow zircon, 
but should more properly be applied to the latter, 
is derived from an old Indian word (cf. p. 229); 
jewellers, however, retain it for the garnet. 

Only the yellow and orange shades of hessonite 
(Plate XXIX, Fig. 5) are used for jewellery. Neither 
the brownish-green kind, to which the term grossular 
may properly be applied, nor the rose-red is trans- 
parent enough to serve as a gem-stone. Hessonite 
may mostly be recognized, even when cut, by the 
curiously granular nature of its structure, just as if 
it were composed of tiny grains imperfectly fused 
together ; this appearance, which is very character- 
istic, may readily be perceived if the interior of the 
stone be viewed through a lens of moderate power. 

The specific gravity varies from 3-55 to 3-66, 
and the refractive index from 1742 to 1*748. The 
hardness is on the whole slightly below that of 
quartz. When heated before a blowpipe it easily 
fuses to a greenish glass. 

The most suitable material is found in some 
profusion in the gem-gravels of Ceylon, in which it 
is mixed up with zircon of an almost identical 
appearance; both are called hyacinth. Hessonites 
from other localities, although attractive as museum 
specimens, are not large and clear enough for cutting 
purposes. Switzerland at one time supplied good 
stones, but the supply has long been exhausted. 

() PYROPE 
(' Cape-Ruby ') 

Often quite ruby - red in colour (Plate XXIX, 
Fig. 6), this variety is probably the most popular of 



GARNET 213 

the garnets. It is strictly a magnesium-aluminium 
garnet corresponding to the formula Mg 3 Al 2 (SiO 4 ) 3 , 
but usually contains some ferrous oxide and thus 
approaches almandine. Both are included among 
the precious garnets. Its name is derived from 
TTvprn-jrof, fire-like, in obvious allusion to its 
characteristic colour. 

Although at its best pyrope closely resembles 
ruby, its appearance is often marred by a tinge of 
yellow which decidedly detracts from its value. 
Pyrope generally passes as a variety of ruby, and 
under such names as ' Cape-ruby,' ' Arizona-ruby,' 
depending on the origin of the stones, commands a 
brisk sale. The specific gravity varies upwards 
from 3*70, depending upon the percentage amount 
of iron present, and similarly the refractive index 
varies upwards from 1740; in the higher values 
pyrope merges into almandine. Its hardness is 
slightly greater than that of quartz, and may be 
expressed on Mohs's scale by the symbol 7^. 

An enormous quantity of small red stones, 
mostly with a slight tinge of yellow, have been 
brought to light at Teplitz, Aussig, and other spots 
in the Bohemian Mittelgebirge, and a considerable 
industry in cutting and marting them has grown 
up at Bilin. Fine ruby-red stones accompany 
diamond in the ' blue ground ' of the mines at 
Kimberley and also at the Premier mine in the 
Transvaal. Similar stones are also found in 
Arizona and Colorado in the United States, and in 
Australia, Rhodesia, and elsewhere. 

Although commonly quite small in size, pyrope 
has occasionally attained to considerable size. Ac- 
cording to De Boodt the Kaiser Rudolph II had one 



214 GEM-STONES 

in his possession valued at 45,000 thalers (about 
6750). The Imperial Treasury at Vienna con- 
tains a stone as large as a hen's egg. Another 
about the size of a pigeon's egg is in the famous 
Green Vaults at Dresden, and the King of Saxony 
has one, weighing 46 8 carats, set in an Order of 
the Golden Fleece. 

(c) RHODOLITE 

This charming pale-violet variety was found at 
Cowee Creek and at Mason's Branch, Macon County, 
North Carolina, U.S.A., but in too limited amount 
to assume the position in jewellery it might other- 
wise have expected. In composition it lies between 
pyrope and almandine, and may be supposed to 
contain a proportion of two molecules of the 
former to one of the latter. Its specific gravity is 
3-84, refractive index 1760, and hardness 7\. It 
exhibits in the spectroscope the absorption-bands 
characteristic of almandine. 



(d) ALMANDINE 
(Carbuncle) 

This variety is iron-aluminium garnet correspond- 
ing to the formula Fe 3 Al 2 (SiO 4 ) 3 , but the com- 
position is very variable. In colour it is deep 
crimson and violet or columbine-red (Plate XXIX, 
Fig. 8), but with increasing percentage amount of 
ferric oxide it becomes brown and black, and opaque, 
and quite unsuitable for jewellery. The name of 
the variety is a corruption of Alabanda in Asia 
Minor, where in Pliny's time the best red stones 



GARNET 2 1 5 

were cut. Almandine is sometimes known as 
Syriam, or incorrectly Syrian garnet, because at 
Syriam, once the capital of the ancient kingdom of 
Pegu, which now forms part of Lower Burma, 
such stones were cut and sold. Crimson stones, 
cut in the familiar en cabochon form and known as 
carbuncles, were extensively employed for enrich- 
ing metalwork, and a half-century or so ago were 
very popular for ornamental wear, but their day has 
long since gone. Such glowing stones are aptly 
described by their name, which is derived from the 
Latin carbunculus, a little spark. In Pliny's time, 
however, the term was used indiscriminately for all 
red stones. It has already been remarked that the 
word spinel has a similar significance. 

The specific gravity varies from 3-90 for trans- 
parent stones to 4*20 for the densest black stones, 
and the refractive index may be as high as r8io. 
Almandine is one of the hardest of the garnets, and 
is represented by the symbol 7| on Mohs's scale. 
The most interesting and curious feature of 
almandine lies in the remarkable and characteristic 
absorption-spectrum revealed when the transmitted 
light is examined with a spectroscope (p. 61). 
The phenomenon is displayed most vividly by the 
violet stones, and is, indeed, the cause of their 
peculiar colour. 

Although a common mineral, almandine of a 
quality fitted for jewellery occurs in comparatively 
few localities. It is found in Ceylon, but not so 
plentifully as hessonite. Good stones are mined in 
various parts of India, and are nearly all cut at 
Delhi or Jaipur. Brazil supplies good material, 
especially in the Minas Novas district of Minas 



216 GEM-STONES 

Geraes, where it accompanies topaz, and Uruguay 
also furnishes serviceable stones. Almandine is 
found in Australia, and in many parts of the 
United States. Recently small stones of good 
colour have been discovered at Luisenfelde in 
German East Africa. 

(e) SPESSARTITE 

Properly a manganese-aluminium garnet corre- 
sponding to the formula Mn 3 Al 2 (SiO 4 ) 3 , this 
variety generally contains iron in both states of 
oxidation. If only transparent and large enough 
its aurora-red colour would render it most accept- 
able in jewellery. Two splendid stones have, in- 
deed, been found in Ceylon (p. 21 1), and good stones 
rather resembling hessonites have been quarried at 
Amelia <ourt House in Virginia, and others have 
come from Nevada ; otherwise, spessartite is un- 
known as a gem-stone. 

The specific gravity ranges from 4*0 to 4*3, and 
the refractive index is about r8i, both characters 
being high ; the hardness is slightly greater than 
that of quartz. 

(/) ANDRADITE 
(Demantoid, Topazolile, ' Olivine ') 

Andradite is strictly a calcium-iron garnet corre- 
sponding to the formula Ca 3 Fe 2 (SiO 4 ) 3 , but as 
usual the composition varies considerably. It is 
named after d'Andrada, a Portuguese mineralogist, 
who made a study of garnet more than a century 
ago. 



GARNET 217 

Once contemptuously styled common garnet, and- 
radite suddenly sprang into the rank of precious 
stones upon the discovery some thirty years ago of 
the brilliant, green stones (Plate XXIX, Fig. 7) in 
the serpentinous rock beside the Bobrovka stream, a 
tributary of the Tschussowaja River, in the Sissersk 
district on the western side of the Ural Mountains. 
The shade of green varies from olive through 
pistachio to a pale emerald, and is probably due to 
chromic oxide. Its brilliant lustre, almost challeng- 
ing that of diamond, and its enormous colour- 
dispersion, in which respect it actually transcends 
diamond, raise it to a unique position among 
coloured stones. Unfortunately its comparative 
softness limits it to such articles of jewellery as 
pendants and necklaces, where it is not likely to be 
rubbed. When first found it was supposed to be 
true emerald, which does actually occur near 
Ekaterinburg, and was termed ' Uralian emerald.' 
When analysis revealed its true nature, it received 
from science the slightly inharmonious name of 
demantoid in compliment to its adamantine lustre. 
Jewellers, however, prefer to designate it ' olivine,' 
not very happily, because the stones usually cut are 
not olive-green and the name is already in extensive 
use in science for a totally distinct species (p. 225); 
they recognized the hopelessness of endeavouring to 
find a market for them as garnets. The yellow 
kind of andradite known as topazolite would be an 
excellent gem-stone if only it were found large and 
transparent enough. Ordinary andradite is brown 
or black, and opaque ; it has occasionally been used 
for mourning jewellery. 

The specific gravity varies from 3'8 to 3-9, being 



2 1 8 GEM-STONES 

about 3*85 for demantoid, which has a high refractive 
index, varying from i'88o to 1-890, and may with 
advantage be cut in the brilliant form. It is the 
softest of the garnets, being only 6^ on Mohs's 
scale. 

(g) UVAROVITE 

This variety, which is altogether unknown in 
jewellery, is a calcium-iron garnet correspond- 
ing mainly to the formula Ca 3 Cr 2 (SiO 4 ) 3 , but with 
some alumina always present, and was named 
after a Russian minister. It has an attractive green 
colour, and is, moreover, hard, being about /| on 
Mohs's scale, but it has never yet come to light 
of a size suitable for cutting. The specific gravity 
is low, varying from 3^4 1 to 3' 5 2. Unlike the 
kindred varieties it cannot be fused by heating 
before an ordinary blowpipe. 



CHAPTER XXIV 
TOURMALINE 

(Rubellite) 

nr^OURMALINE is unsurpassed even by co- 
X rundum in variety of hue, and it has during 
recent years rapidly advanced in public favour, 
mainly owing to the prodigal profusion in which 
nature has formed it in that favoured State, 
California, the garden of the west. Its comparative 
softness militates against its use in rings, but its 
gorgeous coloration renders it admirably fitted for 
service in any article of jewellery, such as a brooch 
or a pendant, in which a large central stone is 
required. Like all coloured stones it is generally 
brilliant-cut in front and step-cut at the back, but 
occasionally it is sufficiently fibrous in structure 
to display, when cut en cabochon, pronounced 
chatoyancy. 

The composition of this complex species has 
long been a vexed question among mineralogists, but 
considerable light was recently thrown on the sub- 
ject by Schaller, who showed that all varieties of 
tourmaline may be referred to a formula of the 
type 1 2Si0 2 . 3 B 2 3 .(9 -^)[(Al,Fe) 2 3 ].34(Fe,Mn ) Ca, 
Mg,K 2 ,Na 2) Li 2 ,H 2 )O].3H 2 O. The ratios of boric 
oxide, silica, and water are nearly constant in all 



220 GEM-STONES 

analyses, but great variation is possible in the 
proportions of the other constituents. Having 
regard to this complexity, it is not surprising to 
find that the range in colour is so great Colourless 
stones, to which the name achroite is sometimes 
given, were at one time exceedingly rare, but they 
are now found in greater number in California. 
Stones which are most suited to jewellery purposes 
are comparatively free from iron, and apparently 
owe their wonderful tints to the alkaline earths ; 
lithia, for instance, is responsible for the beautiful 
tint of the highly prized rubellite, and magnesia, no 
doubt, for the colour of the brown stones of various 
tints. Tourmaline rich in iron is black and almost 
opaque. It is a striking peculiarity of the species 
that the crystals are rarely uniform in colour 
throughout, the boundaries between the differently 
coloured portions being sharp and abrupt, and the 
tints remarkably in contrast. Sometimes the 
sections are separated by planes at right angles to 
the length of the crystal, and sometimes they are 
zonal, bounded by cylindrical surfaces running 
parallel to the same length. In the latter case a 
section perpendicular to the length shows zones of 
at least three contrasting tints. In the Brazilian 
stones the core is generally red, bounded by white, 
with green on the exterior, while the reverse is the 
case in the Californian stones, the core being green 
or yellow, bounded by white, with red on the 
exterior. Tourmaline may, indeed, be found of 
almost every imaginable tint, except, perhaps, the 
emerald green and the royal sapphire-blue. The 
principal varieties are rose-red and pink (rubellite) 
(Plate XXVII, Fig. i), green (Brazilian emerald), 



TOURMALINE 221 

indigo-blue (indicolite), blue (Brazilian sapphire), 
yellowish green (Brazilian peridot) (Plate XXVII, 
Fig. 2), honey-yellow (Ceylonese peridot), violet-red 
(siberite), and brown (Plate XXVII, Fig. 8). The 
black, opaque stones are termed schorl. 

The name of the species is derived from the 
Ceylonese word, turamali, and was first employed 
when a parcel of gem-stones was brought to 
Amsterdam from Ceylon in 1703 ; in Ceylon, 
however, the term is applied by native jewellers to 
the yellow zircon commonly found in the island. 
Schorl, the derivation of which is unknown, is the 
ancient name for the species, and is still used in 
that sense by miners, but it has been restricted by 
science to the black variety. The 'Brazilian 
emerald ' was introduced into Europe in the 
seventeenth century and was not favourably received, 
possibly because the stones were too dark in colour 
and were not properly cut ; that they should have 
been confused with the true emerald is eloquent 
testimony to the extreme ignorance of the characters 
of gem-stones prevalent in those dark ages. 
Achroite comes from the Greek, a%/3oo?, without 
colour. 

To the crystallographer tourmaline is one of the 
most interesting of minerals. If the crystals, which 
are usually prismatic in form, are doubly terminated, 
the development is so obviously different at the two 
ends (Fig. 77) as to indicate that directional character 
in the molecular arrangement, termed the polarity, 
which is borne out by other physical properties. 
Tourmaline is remarkably dichroic. A brown 
stone, except in very thin sections, is practically 
opaque to the ordinary ray, and consequently a 



222 



GEM-STONES 




J 



section cut parallel to the crystallographic axis, i.e. 
to the length of a crystal prismatically developed, 
transmits only the extraordinary ray. Such sections 
were in use for yielding plane-polarized light before 
Nicol devised the calcite prism known by his name 
(cf. p. 44). It is evident that tourmaline, unless very 
light in tint, must be cut with the table facet 
parallel to that axis, because otherwise the stone 
will appear dark and lifeless. The values of the 
extraordinary and ordinary refrac- 
tive indices range between i'6i4 
and i -63 8, and 1-633 and 1*669 
respectively ; the double refraction, 
therefore, is fairly large, amounting 
to O'O25, and, since the ordinary 
exceeds the extraordinary ray, its 
character is negative. The specific 
gravity varies from 3'O to 3 '2. The 
lower values in both characters 
correspond to the lighter coloured 
stones used in jewellery ; the black 
stones, as might be expected from 
their relative richness in iron, are the 
densest. The hardness is only about the same as that 
of quartz, or perhaps a little greater, varying from 7 to 
7-|. It will be noticed that the range of refractivity 
overlaps that of topaz (q.v.\ but the latter has a 
much smaller double refraction, and may thus be 
distinguished (p. 29). Unmounted stones are still 
more easily distinguished, because tourmaline floats 
in methylene iodide, while topaz sinks. The pyro- 
electric phenomenon (cf. p. 82) for which tourmaline 
is remarkable, although of little value as a test in 
the case of a cut stone, is of great scientific interest, 



FIG. 77. Tourma- 
line Crystal. 



TOURMALINE 223 

because it is strong evidence of the peculiar 
crystalline symmetry pertaining to its molecular 
arrangement. Tourmalines range in price from 53. 
to 2Os. a carat according to their colour and quality, 
but exceptional stones may command a higher rate. 
Tourmaline is usually found in the pegmatite 
dykes of granites, but it also occurs in schists and 
in crystalline limestones. Rubellite is generally 
associated with the lithia mica, lepidolite ; the 
groups of delicate pink rubellite bespangling a 
background of greyish white lepidolite are among 
the most beautiful of museum specimens. Mag- 
nificent crystals of pink, blue, and green tourmaline 
have been found in the neighbourhood of Ekaterin- 
burg, principally at Mursinka, in the Urals, Russia, 
and fine rubellite has come from the Urulga River, 
and other spots near Nertschinsk, Transbaikal, 
Asiatic Russia. Elba produces pink, yellowish, 
and green stones, frequently particoloured ; some- 
times the crystals are blackened at the top, and 
are then known locally as 'nigger-heads.' Ceylon 
supplies small yellow stones -the original tourmaline 
which are confused with the zircon of a similar 
colour, and rubellite accompanies the ruby at Ava, 
Burma. Beautiful crystals, green and red, often 
diversely coloured, come from various parts, such as 
Minas Novas and Arassuhy, of the State of Minas 
Geraes, Brazil. Suitable gem material has been 
found in numerous parts of the United States. 
Paris and Hebron in Maine have produced gorgeous 
pink and green crystals, and Auburn in the same 
state has supplied deep-blue, green, and lilac stones. 
Fine crystals, mostly green, but also pink and 
particoloured, occur in an albite quarry near the 



224 GEM-STONES 

Conn River at Haddam Neck, Connecticut. All 
former localities have, however, been surpassed by 
the extraordinary abundance of superb green, 
and especially pink, crystals at Pala and Mesa 
Grande in San Diego County, California. As 
elsewhere, many-hued stones are common. The 
latter locality supplies the more perfectly trans- 
parent crystals. Kunz states that two remarkable 
rubellite crystals were found there, one being 45 
mm. in length and 42 mm. in diameter, and the 
other 56 mm. in length and 24 mm. in diameter. 
Madagascar, which has proved of recent years to 
be rich in gem-stones, supplies green, yellow, and 
red stones, both uniformly tinted and particoloured, 
which in beauty, though perhaps not in size, beat 
comparison with any found elsewhere. 



CHAPTER XXV 
PERIDOT 

THE beautiful bottle-green stone, which from its 
delicate tint has earned from appreciative 
admirers the poetical sobriquet of the evening 
emerald, and which has during recent years crept 
into popular favour and now graces much of the 
more artistic jewellery, is named as a gem-stone 
peridot a word long in use among French jewellers, 
the origin and meaning of which has been forgotten 
but is known to science either as olivine, on 
account of the olive-green colour sometimes 
characterizing it, or as chrysolite. It is of interest 
to note that the last word, derived from xpvvos, 
golden, and Xt'0o9, stone, was in use at the time of 
Pliny, but was employed for topaz and other yellow 
stones, while his topaz, curiously enough, designated 
the modern peridot (cf. p. 1 99), an inversion that has 
occurred in other words. The true olivine must not 
be confused with the jewellers' 'olivine,' which is 
a green garnet from the Ural Mountains (p. 2 1 7). 
Peridot is comparatively soft, the hardness varying 
from 6 1 to 7 on Mohs's scale, and is suitable only 
for articles which are not likely to be scratched ; the 
polish of a peridot worn in a ring would soon 
deteriorate. The choicest stones are in colour a 
lovely bottle-green (Plate XXIX, Fig. 2) of various 
15 " 5 



226 GEM-STONES 

depths; the olive-green stones (Plate XXIX, Fig. 3) 
cannot compare with their sisters in attractiveness. 
The step form of cutting is considered the best for 
peridot, but it is sometimes cut round or oval in 
shape, with brilliant-cut fronts. 

Peridot is a silicate of magnesium and iron, 
corresponding to the formula (Mg,Fe) 2 SiO 4 , ferrous 
iron, therefore, replacing magnesia. To the ferrous 
iron it is indebted for its colour, the pure magnesium 
silicate being almost colourless, and the olive tint 
arises from the oxidation of the iron. The latitude 
in the composition resulting from this replacement 
is evinced in the considerable range that has been 
observed in the physical characters, but the crystal- 
line symmetry persists unaltered ; the lower values 
correspond to the stones that are usually met with 
as gems. Peridot belongs to the orthorhombic 
system of crystalline symmetry, and the crystals, 
which display a large number of faces, are prismatic 
in form and generally somewhat flattened. The 
stones, however, that come into the market for 
cutting as gems are rarely unbroken. The dichroism 
is rather faint, one of the twin colours being slightly 
more yellowish than the other, but it is more pro- 
nounced in the olive-tinted stones. The values of 
the least and greatest of the principal indices of re- 
fraction vary greatly, from 1*650 and r683 to 
1-668 and 1701, but the double refraction, amount- 
ing to 0-033, remains unaffected. Peridot, though 
surpassed by sphene in extent of double refraction, 
easily excels all the ordinary gem-stones in this 
respect, and this character is readily recognizable in 
a cut stone by the apparent doubling of the opposite 
edges when viewed through the table facet (cf. 



PLATE X.\\'U 



t. KUBEI.LJTE 2 - TOURMALINS 





8. TOURMALINE 



5. BALAS-RUBY 



4. BLUE SPINEL 





7 AMETHYST 



10. KIRK-OPAI 





ALEXANDRITE 
(Ky daylight) 




13- ALEXANDRITE 
fRy artificial HsM, 



CHKVSOBERYL 



:EM-STO\E3 



PERIDOT 227 

p. 41). An equally large variation occurs in the 
specific gravity, namely, from 3-3 to 3-5. 

Peridots of deep bottle-green hue command 
moderate prices at the present day, about 303. a 
carat being asked for large stones ; the paler tinted 
stones run down to a few shillings a carat. The 
rate per carat may be very much larger for stones 
of exceptional size and quality. 

Olivine, to use the ordinary mineralogical term, is 
a common and important constituent of certain 
kinds of igneous rocks, and it is also found in those 
strange bodies, meteorites, which come to us from 
outer cosmical space. Except in basaltic lavas, it 
occurs in grains and rarely in well-shaped crystals. 
Stones that are large and transparent enough for 
cutting purposes come almost entirely from the 
island Zebirget or St. John situated on the west 
coast of the Red Sea, opposite to the port of 
Berenice. This island belongs to the Khedive of 
Egypt, and is at present leased to a French 
syndicate. It is believed to be the same as the 
mysterious island which produced the 'topaz' of 
Pliny's time. Magnificent stones have been dis- 
covered here, rich green in colour, and 20 to 30, 
and occasionally as much as 80, carats in weight 
when cut; a rough m~ss attained to the large 
weight of 190 carats. Pretty, light-green stones 
are supplied by Queensland, and peridots of a less 
pleasing dark-yellowish shade of green, and without 
any sign of crystal form, have during recent years 
come from North America. Stones rather similar 
to those from Queensland have latterly been found 
in the Bernardino Valley in Upper Burma, not far 
from the ruby mines. 



CHAPTER XXVI 
ZIRCON 

{Jargoon, Hyacinth, JacintJi) 

ZIRCON, which, if known at all in jewellery, is 
called by its variety names, jargoon and 
hyacinth or jacinth, is a species that deserves greater 
recognition than it receives. The colourless stones 
rival even diamond in splendour of brilliance and 
display of ' fire ' ; the leaf-green stones (Plate XXIX, 
Fig- J 3) possess a restful beauty that commends 
itself; the deep-red stones (Plate XXIX, Fig. 14), if 
somewhat sombre, have a certain grandeur ; and no 
other species produces such magnificent stones of 
golden-yellow hue (Plate XXIX, Fig. 12). Zircon is 
well known in Ceylon, which supplies the world with 
t <he finest specimens, and is highly appreciated by the 
Tmabitants of that sunny isle, but it scarcely finds 
a place in jewellery elsewhere. The colourless 
stones are cut as brilliants, but brilliant-cut fronts 
with step-cut backs is the usual style adopted for the 
coloured stones. 

Zircon is a silicate of zirconium corresponding to 
the formula ZrSiO 4 , but uranium and the rare earths 
are generally present in small quantities. The aurora- 
red variety is known as hyacinth or jacinth, and the 
term jargoon is applied to the other transparent 



ZIRCON 229 

varieties, and especially to the yellow stones. The 
most attractive colours shown by zircon are leaf- 
green, golden-yellow, and deep red. Other common 
colours are brown, greenish, and sky-blue. Colour- 
less stones are not found in nature, but result from 
the application of heat to the yellow and brown 
stones. 

The name of the species is ancient, and comes 
from the Arabic zarqun, vermilion, or the Persian 
zargun, gold-coloured. From the same source in all 
probability is derived the word jargoon through the 
French jargon and the Italian giacone. Hyacinth 
(cf. p. 2 1 1 ) is transliterated from the Greek vdxivOo?, 
itself adapted from an old Indian word; it is in no 
way connected with the flower of the same name. 
The last word has seen some changes of meaning. 
In Pliny's time yellow zircons were indiscriminately 
classified with other yellow stones as chrysolite. 
His hyacinth was used for the sapphire of the 
present day, but was subsequently applied to any 
transparent corundum. Upon the introduction of 
the terms, sapphire and ruby, for the blue and the 
red corundum hyacinth became restricted to the 
other varieties, of which the yellow was the, 
commonest. In the darkness of the Middle Agis 
it was loosely employed for all yellow stones 
emanating from India, and was finally, with increas- 
ing discernment in the characters of gem-stones, 
assigned to the yellow zircon, since it was the 
commonest yellow stone from India. 

Considered from the scientific point of view, zircon 
is by far the most interesting and the most remark- 
able of the gem-stones. The problem presented by 
its characters and constitution is one that still awaits 



230 GEM-STONES 

a satisfactory solution. Certain zircons, which are 
found as rolled pebbles in Ceylon and never show 
any trace of crystalline faces, have very nearly single 
refraction, and the values of the refractive index 
vary from 1*790 to i'84O, and the specific gravity 
is about 4-00 to 4- 14, and the hardness is slightly 
greater than that of quartz, being about 7^-. On 
the other hand, such stones as the red zircons from 
Expailly have remarkably different properties. 
They show crystalline faces with tetragonal 
symmetry, the faces present being four prismatic 
faces mutually intersecting at right angles and four 
inclined faces at each end (Fig. 78). 
They have large double refraction, 
varying from 0*044 to 0*062, which is 
readily discerned in a cut stone (cf. 
p. 41), and the refractive indices are 
high, the ordinary index varying from 
1*923 to 1*931 and the extraordinary 
from 1*967 to 1*993. Since the 
ordinary is less than the extraordin- 
ary index the sign of the double refraction is 
positive. The specific gravity likewise is much 
higher, varying from 4*67 to 4*71. The second 
type, therefore, sinks in molten silver- thallium 
nitrate, whereas the first type floats. The second 
type is also slightly harder, being about 7^ on 
Mohs's scale. By heating either of these types the 
physical characters are not much altered, except that 
the colour is weakened or entirely driven off and 
some change takes place in the double refraction. 
But between these two types may be found zircons 
upon which the effect of heating is striking. They 
seem to contract in size so that the specific gravity 



ZIRCON 231 

increases as much as three units in the first place of 
decimals, and a corresponding increase takes place 
in the refractive indices, and in the amount of double 
refraction. The cause of these changes remains a 
matter of speculation. Evidently a third type of 
zircon exists which is capable of most intimate 
association with either of the other types, and which 
is very susceptible to the effect of heat. It may be 
noted that stones of the intermediate type are 
usually characterized by a banded or zonal structure 
suggesting a want of homogeneity. The theory has 
been advanced that zircon contains an unknown 
element which has not yet been separated from 
zirconium. Zircon of the first type favours green, 
sky-blue, and golden-yellow colours; honey-yellow, 
light green, blue, and red colours characterize the 
second type ; and the intermediate stones are mostly 
yellowish green, cloudy blue, and green. 

It is another peculiarity of zircon that it some- 
times shows in the spectroscope absorption bands 
(p. 61), which were observed in 1866 by Church. 
Many zircons do not exhibit the bands at all, and 
others only display the two prominent bands in the 
red end of the spectrum. 

Of all the gem-stones zircon alone approaches 
diamond in brilliance of lustre, and it also possesses 
considerable ' fire ' ; it can, of course, be readily 
distinguished by its inferior hardness, but a judg- 
ment based merely on inspection by eye might 
easily be erroneous. 

According to Church, who has made a lifelong 
study of zircon, the green and yellowish stones of 
the first variety emit a brilliant orange light when 
being ground on a copper wheel charged with 



232 GEM-STONES 

diamond dust, and the golden stones of the inter- 
mediate type glow with a fine orange incandescence 
in the flame of a bunsen burner ; the latter pheno- 
menon is supposed to be due to the presence of 
thoria. 

The leaf-green stones almost invariably show a 
series of parallel bands in the interior. 

Zircons vary from 53. to 155. a carat, but 
exceptional stones may be worth more. 

By far the finest stones come from Ceylon. 
The colourless stones are there known as ' Matura 
diamonds,' and the hyacinth includes garnet 
(hessonite) of similar colour, which is found with it 
in the same gravels. The stones are always water- 
worn. Small hyacinths and deep-red stones come 
from Expailly, Auvergne, France, and yellowish-red 
crystals are found in the Ilmen Mountains, Oren- 
burg, Russia. Remarkably fine red stones have 
been discovered at Mudgee, New South Wales, and 
yellowish-brown stones accompany diamond at the 
Kimberley mines, South Africa. 



CHAPTER XXVII 
CHRYSOBERYL 

(Chrysolite, Cats -Eye, Cymophane, Alexandrite) 

CHRYSOBERYL has at times enjoyed fleeting 
popularity on account of the excellent cat's- 
eyes cut from the fibrous stones, and in the form of 
alexandrite it meets with a steadier, if still limited, 
demand. It is a gem-stone that is seldom met with 
in ordinary jewellery, although its considerable 
hardness befits it for all such purposes. 

Chrysoberyl is in composition an aluminate of 
beryllium corresponding to the formula BeAl 2 O 4 , 
and is therefore closely akin to spinel. It usually 
contains some ferric and chromic oxides in place of 
alumina, and ferrous oxide in place of beryllia, and 
it is to these accessory constituents that its tints are 
due. Other gem-stones containing the uncommon 
element beryllium are phenakite and beryl. Pale 
yellowish green, the commonest colour, is supposed 
to be caused by ferrous oxide ; such stones are known 
to jewellers as chrysolite (Plate XXVII, Fig. 12). 
Cat's-eyes (Plate XXIX, Fig. i) have often also a 
brownish shade of green. The bluish green and dark 
olive-green stones known as alexandrite (Plate XXVII, 
Figs. II, 13) differ in appearance so markedly from 
their fairer sisters that their common parentage seems 



234 GEM-STONES 

almost incredible. The dull fires that glow within 
them, and the curious change that comes over them 
at night, add a touch of mystery to these dark 
stones. Chromic oxide is held responsible for their 
colour. The cat's-eyes are, of course, always cut en 
cabochon, but otherwise chrysoberyl is faceted. 

The name of the species is composed of two 
Greek words, xpvcros, golden, and /3ijpv\\o<;, beryl, 
and etymologically more correctly defines the lighter- 
coloured stones, which were, indeed, at one time 
the only kind known. Chrysolite from ^puo-o?, 
golden, and \/0o9, stone, has much the same signifi- 
cance. This name is preferred by jewellers, but in 
science it is applied to an entirely different species, 
which is known in jewellery as peridot. Cymo- 
phane, from Kvpa, wave, and <f>aiveiv, appear, refers 
to the peculiar opalescence characteristic of cat's- 
eyes ; it is sometimes used to designate these stones, 
but does not find a place within the vocabulary of 
jewellery. Alexandrite is named after Alexander 
II, Czar of Russia, because it first came to light on 
his birthday. That circumstance, coupled with its 
display of the national colours, green and red, and 
its at one time restriction to the mining district near 
Ekaterinburg, renders it dear to the heart of all 
loyal Russians. 

Chrysoberyl crystallizes in the orthorhombic 
system, and occurs in rather dull, complex crystals, 
which are sometimes so remarkably twinned, especi- 
ally in the variety called alexandrite, as to simulate 
hexagonal crystals. In keeping with the crystalline 
symmetry it is doubly refractive and biaxial, having 
two directions of single refraction. The least and 
the greatest of the principal indices of refraction 



CHRYSOBERYL 235 

may have any values between 1742 and 1749, 
and 1750 and 1757, respectively, the maximum 
amount of double refraction remaining always the 
same, namely, 0*009. The mean principal refractive 
index is close to the least ; the sign of the double 
refraction is therefore positive, and the shadow-edge 
corresponding to the lower index, as seen in the 
refractometer, has little, if any, perceptible motion 
when the stone is rotated. The converse is the 
case with corundum ; the sign is negative, and it is 
the shadow-edge corresponding to the greater re- 
fractive index that remains unaltered in position on 
rotation of the stone. This test would suffice to 
separate chrysoberyl from yellow corundum, even if 
the refractive indices of the former were not sensibly 
lower than those of the latter. Also, the dichroism 
of chrysolite is stronger than that of yellow 
sapphires. In alexandrite this phenomenon is most 
prominent; the absorptive tints, columbine-red, 
orange, and emerald-green, corresponding to the 
three principal optical directions, are in striking con- 
trast, and the first differs so much from the intrinsic 
colour of the stone as to be obvious to the unaided 
eye, and is the cause of the red tints visible in a cut 
stone. The curious change in colour of alexandrite, 
from leaf-green to raspberry-red, that takes place 
when the stone is seen by artificial light, is due to 
a different cause, as has been pointed out above 
(p. 54). The effect is illustrated by Figs. 1 1, 13 on 
Plate XXVII, which represent a fine Ceylon stone as 
seen by daylight and artificial light; the influence 
of dichroism may be noticed in the former picture. 
The specific gravity of chrysoberyl varies from 3'68 
to 378. In hardness this species ranks above spinel 



236 GEM-STONES 

and comes next to corundum, being given the 
symbol 8J on Mohs's scale. Certain stones contain 
a multitude of microscopic channels arranged in 
parallel position. When the stones are cut with 
their rounded surface parallel to the channels, a 
broadish band of light is visible running across the 
stone at right angles to them, and suggests the pupil 
of a cat's eye, whence the common name for the 
stones. The fact that the channels are hollow 
causes an opalescence, which is absent from the 
quartz cat's-eye. 

The most important locality for the yellowish 
chrysoberyl is the rich district of Minas Novas, 
Minas Geraes, Brazil, where it occurs in the form of 
pebbles, and excellent material is also supplied by 
Ceylon, in both crystals and rounded pebbles. 
Other places for chrysolite are Haddam, Connecti- 
cut, and Greenfield, Saratoga County, New York, 
in the United States, and recently in the gem- 
gravels near the Somabula Forest, Rhodesia. 
Ceylon supplies some of the best cat's-eyes. Alex- 
andrite was first discovered, as already stated, at the 
emerald mines near Ekaterinburg, in the Urals ; but 
the supply is now nearly exhausted. A poorer 
quality comes from Takowaja, also in the Urals. 
Good alexandrite has come to light in Ceylon, and 
most of the stones that are placed on the market at 
the present day have emanated from that island. 
The Ceylon stones reach a considerable size, often 
as much as from 10 to 20 carats in weight; the 
Russian stones have a better colour and are more 
beautiful, but they are less transparent, and rarely 
exceed a carat in weight. Good chrysolite may be 
worth from IDS. to 2 a carat, and cat's-eye runs 



CHRYSOBERYL 237 

from 1 to 4 a carat, depending upon the quality. 
Alexandrites meet with a steady demand in Russia, 
and fine stones are scarce ; flawless stones about a 
carat in weight are worth as much as 30 a carat, 
and even quite ordinary stones fetch 4. a carat. 

From Ceylon, that interesting home of gems, have 
originated some magnificent chrysoberyls, including 
a superb chrysolite, 8of carats in weight, and 
another, a splendid brownish yellow in colour and 
very even in tint, and two large alexandrites, green 
in daylight and a rich red by night, weighing 63! 
and 28|-f carats. The finest cut chrysolite existing 
is probably the one exhibited in the Mineral Gallery 
of the British Museum (Natural History). Abso- 
lutely flawless and weighing 43! carats, it was 
formerly contained in the famous Hope collection, and 
is described on page 56 and figured on Plate XXI 
of the catalogue prepared by B. Hertz, which was 
published in 1839 ; the weight there given includes 
the brilliants and the ring in which it was mounted. 
It is shown, about actual size, in Plate XXVII, Fig. 1 2. 
A magnificent cat's-eye, 3 5 '5 by 3 5 mm. in size, which 
also formed part of the Hope collection, was included 
in the crown jewels taken from the King of Kandy 
in 1815. The crystalline markings in the cut stone 
are so arranged that the lower half shows an altar 
overhung by a torch. The stone has been famous 
in Ceylon for many ages. It was set in gold with 
rubies cut en cabochon. Two fine Ceylon alexand- 
rites of exceptional merit, weighing 42 and 26f 
carats, are also exhibited in the Mineral Gallery of 
the British Museum (Natural History). The former 
is illustrated in Plate XXVII, Figs. 1 1, 1 3, as seen in 
daylight and in artificial light. 



CHAPTER XXVIII 
QUARTZ 

(Rock-Crystal, Amethyst, Citrine, Cairngorm, Cafs- 
Eye, Tigers-Eye) 

A LTHOUGH the commonest and, in its natural 
\. form, the most easily recognizable of mineral 
substances, quartz nevertheless holds a not incon- 
spicuous position among gem-stones, because, as 
amethyst (Plate XXVII, Fig. 7), it provides stones of 
the finest violet colour ; moreover, the yellow quartz 
(Plate XXVII, Fig. 5) so ably vies with the true topaz 
that it is universally known to jewellers by the name 
of the latter species, and is too often confounded 
with it, and the lustrous, limpid rock-crystal even 
aspires to the local title of ' diamond.' For all 
purposes where a violet or yellow stone is required, 
quartz is admirably suited ; it is hard and durable, 
and it has the merit, or possibly to some minds the 
drawback, of being moderate in price. Despite its 
comparative lack of ' fire,' rock-crystal might replace 
paste in rings and buckles with considerable advan- 
tage from the point of view of durability. The 
chatoyant quartz, especially in the form known as 
tiger's-eye, will for beauty bear comparison with the 
true cat's-eye, which is a variety of chrysoberyl. 
Except that cat's-eye is cut en cabochon, quartz is 

step- or sometimes brilliant-cut. 
238 



QUARTZ 239 

Ranking with corundum next to diamond as the 
simplest in composition of the gem-stones, quartz is 
the crystallized form of silica, oxide of silicon, corre- 
sponding to the formula SiO 2 . When pure, it is 
entirely devoid of the faintest trace of colour and 
absolutely water- clear. Such stones are called rock- 
crystal, and it is easy to understand why in early 
days it was supposed to represent a form of petrified 
water. It is these brilliant, transparent stones that 
are, when small, known in many localities as 
'diamonds.' Before the manufacture of glass was 
discovered and brought to perfection, rock-crystal 
was in considerable use for fashioning into cups, 
vases, and so forth. The beautiful tints character- 
izing quartz are due to the usual metallic oxides. 
To manganese is given the credit of the superb 
purple or violet colour of amethyst, which varies 
considerably in depth. Jewellers are inclined to 
distinguish the deep-coloured stones with the prefix 
1 oriental,' but the practice is to be deprecated, since 
it might lead to confusion with the true oriental 
amethyst, which is a purple sapphire, one of the 
rarest varieties of corundum. Quartz of a yellow 
hue is properly called citrine, but, as already stated, 
jewellers habitually prefer the name ' topaz ' for it, 
and distinguish the true topaz by the prefix 
Brazilian not a very happy term, since both the 
yellow topaz and the yellow quartz occur plentifully 
in Brazil. Sometimes the yellow quartz is termed 
occidental, Spanish, or false topaz. Stones with a 
brownish or smoky tinge of yellow are called 
cairngorm, or Scotch topaz. The colour of the 
yellow stones is doubtless due to a trace of ferric 
oxide. Stones of a smoky brown colour are known 



240 GEM-STONES 

as smoky-quartz. Rose-quartz, which is rose-red or 
pink in colour and hazy in texture, is comparatively 
rare ; strange to say, it has never been found in 
distinct crystals. The tint, which may be due to 
titanium, is fugitive, and fades on exposure to strong 
sunlight. In milky quartz, as the name suggests, 
the interior is so hazy as to impart to the stone a 
milky appearance. It has frequently happened that 
quartz has crystallized after the formation of other 
minerals, with the result that the latter are found 
inside it. Prase, or mother-of-emerald, which at one 
time was supposed to be the mother-rock of emerald, 
is a quartz coloured leek-green by actinolite fibres 
in the interior. Specimens containing hair-like 
fibres of rutile the so-called fleches d'amour are 
common in mineral collections, and are sometimes 
to be seen worked. When enclosing a massive, 
light-coloured, fibrous mineral, the stones have a 
chatoyant effect, and display, when suitably cut, a 
fine cat's-eye effect ; in tiger's-eye the enclosed 
mineral is crocidolite, an asbestos, the original blue 
hue of which has been changed to a fine golden- 
brown by oxidation. Quartz which contains scales 
of mica, hematite, or other flaky mineral has a vivid 
spangled appearance, and is known as aventurine ; 
it has occasionally been employed for brooches or 
similar articles of jewellery. Rainbow-quartz, or 
iris, is a quartz which contains cracks, the chromatic 
effect being the result of the interference of light 
reflected from them ; it has been artificially produced 
by heating the stone and suddenly cooling it. 

The name of the species is an old German mining 
term of unknown meaning which has been in general 
use in all languages since the sixteenth century. 



QUARTZ 241 

Amethyst is derived from apeOva-ros, not drunken, 
possibly from a foolish notion that the wearer was 
exempt from the usual consequences of unrestrained 
libations. Pliny suggests as an alternative explana- 
tion that its colour approximates to, but does not 
quite reach, that of wine. Aventurine, from aventura, 
an accident, was first applied to glass spangled with 
copper, the effect being said to have been acci- 
dentally discovered owing to a number of copper 
filings falling into a pot of molten glass in a Venetian 
factory. 

Quartz belongs to the hexagonal system of 
crystalline symmetry, and crystal- 
lizes in the familiar six-sided prisms 
terminated by six inclined, often 
triangular, faces (Fig. 79) ; twins are 
common, though they are not always 
obvious from the outward develop- 
ment. In accordance with the sym- _ 

e L . . ill FlG - 79- Quartz 

metry the refraction is double, and Crystal. 

there is one direction of single re- 
fraction, namely, that parallel to the edge of the 
prism. The ordinary refractive index has the 
value i '544, and the extraordinary i'5S3, and since 
the latter is the greater, the sign of the double 
refraction is positive. The double refraction is 
small in amount, but is large enough to enable 
the apparent doubling of certain of the opposite 
edges of a faceted stone to be perceptible when 
viewed with a lens through the table-facet. The 
dichroism of the deep-coloured stones is quite 
distinct. Quartz has only about the same amount 
of colour dispersion as ordinary glass, and lacks, 
therefore, 'fire.' The application of strong heat 
16 



242 GEM-STONES 

tends, as usual, to weaken or drive off the colour. 
Thus the dense smoky-quartz found in Spain, Brazil, 
and elsewhere is converted into stones of a colour 
varying from light yellow to reddish brown accord- 
ing to the amount and duration of the application. 
In the case of amethyst the colour is changed to a 
deep orange, or entirely driven off if the temperature 
be high enough. Its density is very constant, vary- 
ing only from 2*654 to 2'66o ; the purest stones 
are the lightest. To it has been assigned the symbol 
7 on Mohs's scale of hardness. 

To physicists quartz is one of the most interesting 
of minerals because of its power of rotating, to an 
extent depending upon the thickness of the section, 
the plane of polarization of a beam of light tra- 
versing it in a direction parallel to the prism edge. 
It appears, moreover, from a study of the pyro- 
electric and general physical characters, that its 
molecular structure has a helical arrangement, which, 
like all screws, may have a right- or left-handed 
character. Amethyst is, in fact, invariably composed 
of separate twin individuals, alternately right- and 
left-handed ; in some remarkable crystals the section 
at right angles to the prism edge is composed of 
triangular sectors, alternately of different hands 
and of different tints purple and white. To the 
twinning is due the rippled fracture and the feathery 
inclusions so characteristic of amethyst. 

Besides its use for ornamental purposes, quartz 
finds a place as the material for lenses intended for 
delicate photographic work, because its transparency 
to the ultra-violet light is so much greater than that 
of glass. Spectacle lenses made of it are in demand, 
because they are not liable to scratches, and retain, 



QUARTZ 243 

therefore, their polish indefinitely. When fused in the 
oxyhydrogen flame, quartz becomes a silica glass, of 
specific gravity 2 -2 and hardness 5 on Mohs's scale, 
which has proved of great service for laboratory 
ware, because it withstands sudden and unequal 
heating without any danger of fracture ; it has also 
in fine threads been invaluable for delicate torsion 
work, because it acquires not the smallest amount of 
permanent twist, in this respect being superior to 
the finest silk threads. 

Clear rock-crystal fetches little more than the 
cost of the cutting ; citrine and amethyst are worth 
from is. to 53. a carat, depending upon the quality 
and size of the stone; smoky-quartz is practically 
valueless; rose-quartz realizes less than is. a carat ; 
and the value of cat's-eye is also small only is. 
to 2s. 6d. a carat. Tiger's-eye at one time com- 
manded as much as 253. a carat, but the supply 
exceeded the demand, with the consequent collapse 
in the price. 

Beautiful, brilliant, and limpid rock-crystal is 
found in various parts of the world : in the Swiss 
Alps, at Bourg d'Oisans in the Dauphine" Alps, 
France, in the famous Carrara marble, in the Mar- 
maros Comitat of Hungary, and in the United 
States, Brazil, Madagascar, and Japan. Small 
lustrous stones, known in their localities as ' Isle 
of Wight,' ' Cornish,' or ' Bristol diamonds,' are found 
in our own country. Brazil supplies stones out 
of which have been cut the clear balls used 
in crystal-gazing. The finest amethysts come 
from Brazil especially the State of Rio Grande 
do Sul and from Uruguay, India, and the gem- 
gravels of Ceylon; good stones also occur at 



244 GEM-STONES 

Ekaterinburg, in the Ural Mountains. A splendid 
Brazilian amethyst, weighing 334 carats, and two 
Russian stones one hexagonal in contour, weighing 
88 carats, and the other, a deep purple in colour 
with a circular table, weighing 73 carats are 
exhibited in the British Museum (Natural History). 
Cairngorm is known from the place of that name 
in Banffshire, Scotland, whence fine specimens have 
emanated ; it is a gem much valued in that country. 
Fine cairngorm has also originated from Pike's 
Peak, Colorado. Splendid yellow stones have had 
their birth in the States of Minas Geraes, Sao Paulo, 
and Goyaz, of Brazil especially in the last. The 
fine Spanish smoky-quartz, which, as already stated, 
turns yellow on heating, comes from Hinojosa, in 
the Province of Cordova. The delicate rose-quartz 
is known at Bodenmais in Bavaria, Paris in Maine, 
United States, and Ekaterinburg in the Ural 
Mountains. The finest cat's-eyes are found in India 
and Ceylon, and are high in favour with the natives. 
Greenish stones of an inferior quality are brought 
from the Fichtelgebirge in Bavaria, and are sold as 
' Hungarian cat's-eyes,' despite the fact that no 
such stone occurs in Hungary another instance of 
jewellers' disdain for accuracy. Tiger's-eye occurs 
in considerable quantity in the neighbourhood of 
Griquatown, Griqualand West, South Africa. A 
silicified ctocidolite, in which the blue colour is 
retained, comes also from Salzburg, and is known as 
sapphire- or azure-quartz, or siderite. 

Certain of the pebbles found on the seashore of 
our coasts, especially off the Isle of Wight and 
North Wales, cut into attractive, clear stones, more 
or less yellow in colour ; but examples suitable for 



QUARTZ 245 

the purpose are not so numerous as might be 
supposed, and do not reward any casual search. 
Les affaires sont les affaires. The local lapidary, 
instead of explaining that the pebbles brought to 
him are not worth cutting, finds it more convenient 
and profitable to substitute for them other, inferior 
and badly .cut, stones, bought by the gross, or even 
paste stones ; the customer, on the other hand, is 
contented with a pretty bauble, and is not grateful 
for the information that it might have been obtained 
for a fraction of the sum paid. 



CHAPTER XXIX 
CHALCEDONY, AGATE, ETC. 

/CHALCEDONY and agate, and their endless 

V ' varieties, are composed mainly of silica, but 

the separate individual crystals are so small as 
to be invisible to the unaided eyesight, and occasion- 
ally are so extremely minute that the structure is 
almost amorphous. The colour and appearance vary 
greatly, depending upon the impurities contained in 
the stone, and, since both have been made a criterion 
for differentiation of types, a host of names have 
come into use, none of which are susceptible of 
strict definition. On the whole, these stones may 
be divided into two groups : chalcedony, in which 
the structure is concretionary and the colour 
comparatively uniform, and agate, in which the 
arrangement takes the form of bands, varying 
greatly in tint and colour. 

The refraction, though double in the individual, 
is irregular over the stone as a whole, and the indices 
approximate to 1*550. The specific gravity ranges 
from 2*62 to 2'64, depending upon the impurities 
present. The degree of hardness is about the same as 
that of quartz, namely, 7 on Mohs's scale. All kinds 
are more or less porous, and stones of a dull colour 
are therefore artificially tinted after being worked. 

The term chalcedony, derived from ^a\K^atv 
246 



CHALCEDONY, AGATE, ETC. 247 

the name of a town in Asia Minor, is usually 
confined to stones of a greyish tinge. Stones 
artifically coloured an emerald green have been cut 
and put upon the market as ' emeraldine.' Carnelian 
is a clear red chalcedony, and sard is somewhat 
similar, but brownish in tint. Chrysoprase is apple- 
green in colour, nickel oxide being supposed to be 
the agent. Prase (cf. p. 240), which is a dull leek- 
green in hue, may also in part be referred here ; 
the name comes from irpdcrpov, a leek. Plasma, 
which may have the same derivation, is a brighter 
leek-green. Jasper is a chalcedony coloured blood- 
red by iron oxide, while bloodstone is a green 
chalcedony spotted with jasper; they are popular 
stones for signet rings. Flint, an opaque 
chalcedony, breaks with a sharp cutting edge, and 
was much in request with early man as a tool or a 
weapon ; its property of giving sparks when struck 
with steel rendered it invaluable before the invention 
of matches. Hornstone is somewhat similar, but 
more brittle, while chert is a flinty rock. 

Agate, named after the river Achates in Sicily, 
where it was found at the time of Theophrastus, 
has a peculiar banded structure, the bands being 
usually irregular in shape, following the configura- 
tion of the cavity in which it was formed. Moss- 
agate, or mocha-stone, contains moss-like inclusions 
of some fibrous mineral. Onyx is an agate with 
regular bands, the layers having sharply different 
colours ; when black and white, it has, in days gone 
by, been employed for cameos. Sardonyx is similar 
in structure, but red and white in colour. Agate is 
used in delicate balances for supporting the steel 
knife-edges of the balance itself and of the pan- 



248 GEM-STONES 

holders, and is largely employed especially when 
artificially coloured for umbrella handles and similar 
articles. 

Chalcedony and agate are found the whole world 
over, but India, and particularly Brazil, are noted 
for their fine carnelians and agates. 



CHAPTER XXX 
OPAL 

(White Opal, Black Opal, Fire-Opal) 

THAT opal in early times excited keen 
admiration is evident from Pliny's enthusi- 
astic description of these stones : " For in them 
you shall see the burning fire of the carbuncle, the 
glorious purple of the amethyst, the green sea of 
the emerald, all glittering together in an incredible 
mixture of light." During much of last century, 
owing to the foolish superstition that ill-luck dogs 
the footsteps of the wearer, the species lay under a 
cloud, which has even now not quite dispersed, but 
exercises a prejudicial effect upon the fortunes of 
the stone. It has, however, recently attracted 
considerable attention owing to the discovery of the 
splendid black opals in Australia ; at one moment 
black with the darkness of night, at the next by a 
chance movement glowing with vivid crimson 
flame, such stones may justly be considered the 
most remarkable in modern jewellery. At the 
present day opal is divided by jewellers roughly 
into two main groups : ' white ' (Plate XXVII, Fig. 6) 
and ' black' (Plate XXVII, Fig. 9), according as the 
tint is light or dark, fire-opal (Plate XXVII, Fig. 10) 
standing in a separate category. 



250 GEM-STONES 

Opal differs from the rest of the principal gem- 
stones in being not a crystalline body, but a 
solidified jelly, and it depends for its attractiveness 
upon the characteristic play of colour, known, in 
consequence, as opalescence (cf. p. 39), which 
arises from a peculiarity in the structure. Opal is 
mainly silica, SiO 2 , in composition, but contains in 
addition an amount of water varying in precious 
opal from 6 to 10 per cent. As the original jelly 
cooled, it became riddled throughout with cracks, 
which were afterwards generally filled with opal 
matter, containing a different amount of water, and 
therefore differing slightly in refractivity from the 
original substance. The structure not being quite 
homogeneous, each crack has the same action upon 
light as a soap-film, and gives rise to precisely 
similar phenomena ; the thinner and more uniform 
the cracks, the greater the splendour of the chromatic 
display, the particular tint depending upon the 
direction in which the stone is viewed. The cracks 
in certain opals were not filled up, and therefore 
contain air. Such stones appear opaque and devoid 
of opalescence until plunged into water ; they are 
consequently known as hydrophane, from vScop, water, 
and <j>aive<r0ai, to make appear. Owing to the effect 
of total -reflection, light was stopped on the hither 
side of the cracks before they were filled with water, 
which is not far inferior to opal in refractivity; it 
is surprising how much water these stones will 
absorb. 

Opal is colourless when pure, but is nearly 
always more or less milky and opaque, or tinted 
various dull shades by ferric oxide, magnesia or, 
alumina. The so-called black opal is generally a 



OPAL 251 

dark grey or blue, and very rarely quite black. 
That the coloration is not due to ordinary absorp- 
tion, but to the action of cracks in the stone, is 
shown by the fact that the transmitted light is 
complementary to the reflected light ; the blue 
opal is, for instance, a yellow when held up so that 
light has passed through it. In many black opals 
the opalescent material occurs in far too tiny pieces 
to be cut separately, and the whole iron-stained 
matrix is cut and polished and sold under the name 
' opal-matrix.' The reddish and orange-coloured 
stones known as fire-opal have pronounced colour 
and only slight milkiness ; they display the 
customary opalescence in certain directions. These 
stones are often faceted, but otherwise opals are 
cut en cabochon, either flat or steep generally the 
former in brooches and pendants, and the latter in 
rings. Opal is somewhat soft, varying from 5 to 6 
on Mohs's scale, and is therefore easily scratched. 
The specific gravity ranges from 2*10 to 2-20, and 
the refractive index from 1*444 to 1*464, the 
refraction, of course, being always single. It is 
unwise to immerse opals in liquids on account of 
their porosity. 

The name opal comes to us through the Latin 
opallus, which was used for the same species as 
understood by the term at the present day, but the 
word has a far older origin, which has not been 
traced. The Romans also called the mineral 
pczderos, the Greek form of Cupid, a name applied 
to all rosy stones. The name cacholong, for the 
bluish-white procelain variety, which is very porous 
and adheres to the tongue, is of Tartar origin ; the 
stone is highly valued in the East. 



252 GEM-STONES 

The oldest mines, which up to quite a recent 
date were the only extensive deposit of opal known, 
were at Cserwenitsa, near Kashau, in Hungary. 
From them in all probability emanated the opals 
known to the Romans. The opals from this locality 
were generally quite small, and large pieces were 
rare and commanded high prices. The Hungary 
mines, however, proved quite unable to compete 
with the rich fields at White Cliffs, New South 
Wales, in spite of the efforts that were made to 
depreciate and exclude from the market the new 
stones, and at the present time few of the opals on 
the market come from them. As so often happens, 
the White Cliffs deposit was discovered by accident. 
In 1889 a hunter, when tracking a wounded 
kangaroo, chanced to pick up an attractively coloured 
opal. The district is so waterless and forbidding 
that, but for such a chance, the opals might have 
long lain hidden. They occur in seams in deposits 
of Cretaceous Age in a variety of ways, filling 
cavities in rocks or sandstones, or cracks in wood, 
or replacing wood, saurian bones, and some spiky 
mineral, which may have been glauberite. In 
recent years, another rich deposit was discovered 
farther north, on both sides of the boundary between 
Queensland and New South Wales. The field is- 
remarkable for the darkness of its opals, which are 
called ' black opal ' in contradistinction to the 
lighter-coloured stones previously known. From 
Lightning Ridge in New South Wales come stones 
stained deep black which quite merit the designa- 
tion black opal. The sandstone in which they 
are found is rich in iron, and this is no doubt re- 
sponsible for the deepness of their tint. Mexico is 



PL A TE XX VII I 




OPAL 253 

noted for the fire-opal, which is found at Esperanza, 
Queretaro, and Zimapan ; but other kinds of opal 
also are found at these places. 

The price of opal varies greatly, according to 
the intrinsic colour and the uniformity and brilliance 
of the opalescence. Common opal can be bought 
at as low a rate as is. a carat, while black opal 
ranges from I os. to 8 a carat ; but a good dark 
stone displaying a flaming opalescence commands 
a fancy figure, fine stones of this class being ex- 
ceedingly rare. Fire-opal enjoys only a limited 
popularity now, though a few years ago it was in 
some demand; the price runs from 2s. to los. a 
carat. 



CHAPTER XXXI 
FELSPAR 

(Moonstone, Sunstone, Labradorite, Amazon- Stone) 

THOUGH second to none among minerals in 
scientific interest, whether regarded from 
the point of view of their crystalline characters or 
the important part they play in the formation of 
rocks, the group included under the general name 
felspar occupies but a humble place in jewellery. It 
consists of three distinct species, orthoclase, albite, 
and anorthite, which are silicates of aluminium, and 
potassium, sodium, or calcium, corresponding to 
the formulae KAlSi 3 O 8 , NaAlSi 3 O 8 , and CaAl 2 Si 2 O 8 
respectively, and also of species intermediate in 
composition between albite and orthoclase, or albite 
and anorthite. While differing in crystalline 
symmetry, all are characterized by two directions 
of cleavage which are nearly at right angles to one 
another. The double refraction, which is slight in 
amount, is biaxial in character and variable in sign. 
The values of the least and greatest of the indices 
of refraction range between 1*52 and i'53, and 1*53 
and I '5 5 respectively, the double refraction at the 
same time varying from 0*007 to 0*012. The 
specific gravity lies between 2*48 and 2*66, and 



FELSPAR 255 

the hardness ranges between the degrees 6 and 7 
on Mohs's scale. 

Moonstone (Plate XXIX, Fig. 4), which is mainly 
pure orthoclase, alone is at all common in jewellery. 
It forms such an admirable contrasting frame for 
large coloured stones that it deserves greater 
popularity ; no doubt the cheapness of the stones 
militates against their proper appreciation. The 
milky, bluish opalescence from which they take 
their name is caused by the reflection of light at 
the thin twin-lamellae of which the structure is 
composed. They are always cut more or less 
steeply en cabochon. The finest stones were at one 
time cut from the felspar that came from the St. 
Gothard district in Switzerland and was in con- 
sequence known as adularia from the neighbouring 
Adular Mountains, somewhat incorrectly, since none 
occurs at the latter locality. At the present day 
practically all the moonstones on the market come 
from Ceylon. They run in price from 3 to 20 
per oz. (28 grams). 

Sunstone is a felspar containing flakes of hematite 
or goethite which impart a spangled bronze appear- 
ance to the stones. Good material occurs in parts 
of Norway. The remarkable sheen of labradorite 
or blue felspar has its origin in the interference of 
light at lamellar surfaces in the interior ; the uni- 
formity of the colour over comparatively large areas 
testifies to the regularity of the lamellar arrange- 
ment. The finest specimens were brought from 
the Isle of St. Paul off the coast of Labrador, where 
they were first discovered in 1770; large masses 
also occur on the coast itself. Amazon-stone is an 
opaque green felspar which occurs in the Ilmen 



256 GEM-STONES 

Mountains, Orenburg, Russia, and at Pike's Peak, 
Colorado, United States. It obtains its name from 
the Amazon River, where, however, none has ever 
been found ; there may have been some confusion 
with a jade or similar stone. 

Occasionally clear colourless felspar has been 
faceted, and then closely resembles rock-crystal. 
A careful determination of the refractive indices 
and the specific gravity serves to discriminate 
between them 



PLATE .\.\IX 






4. MOONSTONE 





S. HESSONJTE 5. I'YROl'E 








[I. IIIDDENITE 



14. ZIRCON 15. ANDAI.US1TE 




10. NEPHRIT! 




(1KM-STONICS 



CHAPTER XXXII 
TURQUOISE, ODONTOLITE, VARISC1TE 

OF all the opaque stones turquoise (Plate XXIX, 
Fig. 17) alone finds a prominent place in 
jewellery and can aspire to rank with the precious 
stones. The colour varies from a sky-blue or a 
greenish blue to a yellowish green or apple-green. 
Only the former tints, which are at the same time 
the rarer, are in general demand, and they possess 
the great advantage of harmonizing with the tint 
of the gold setting. The blue colours are, especially 
in the case of the Siberian stones, by no means 
permanent, and fade in course of time. Turquoise 
is amorphous and seldom crystalline, and is therefore 
somewhat porous ; it should consequently never 
be immersed in liquids or be contaminated with 
greasy and dirty matter lest the dreaded change 
of colour be brought about. The stones are trans- 
lucent in thin sections, and a good observation is 
possible with the refractometer if the back of the 
stone is flat and polished, since only the section 
immediately adjacent to the instrument is concerned ; 
the refractive index is about r6.l. The specific 
gravity varies from 275 to 2-89. Turquoise has 
a hardness of slightly under 6 on Mohs's scale, 
and takes a good polish, which is fairly durable, 
since on account of the comparative opacity of the 
17 2 S7 



258 GEM-STONES 

stones scratches on the surface are not very notice- 
able. In composition it is a complex phosphate of 
aluminium and copper, corresponding to the formula 
CuOH.[6Al(OH) 2 ].H 5 .(PO 4 ) 4 , with ferric oxide replac- 
ing some alumina The blue colour is due to the 
copper constituent, and the predominance of iron 
may cause the greenish shades ; but the water 
contained in the stones plays no mean part, since 
they turn a dirty green when it is driven off 
The faded colour can sometimes be restored by 
immersion of the stone in ammonia and subse- 
quent application of grease, but the effect is not 
lasting. Attempts are sometimes made to improve 
inferior stones by impregnating them with Berlin 
blue, but with only qualified success. Turquoises 
are said to be affected by the perspiration from 
the skin. 

The name of the species comes from a French 
word meaning Turkish, and arises from the fact that 
the gem-stone first reached Europe by way of Turkey. 
Another, but less obvious, suggestion is that it is 
derived from the Persian name for the species, 
piruzeh. Our turquoise and other phosphates of 
similar appearance were probably known to Pliny 
under the three names callais, callaina> and callaica. 

The finest turquoise still comes from the famous 
mines near Nishapur in the Persian province of 
Khorassan, where it was known in very ancient 
times; it is found with limonite filling the cracks 
and cavities in a brecciated porphyritic trachyte. 
Pieces of the turquoise and limonite from here are 
sometimes cut without removal of the latter, and 
sold as ' turquoise-matrix,' when the precious stones 
are too tiny to be worth separate working. It also 



TURQUOISE, ODONTOLITE, VARISCITE 259 

occurs at Serbal in the Sinai Peninsula. Among 
the more recent localities may be mentioned Los 
Cerillos Mountains, New Mexico; Sierra Nevada, 
Nevada, where pale blue and green stones are 
found ; San Bernardino County, California, where 
again the stones are rather pale ; and Arizona, 
where it occurs in pale greenish-blue stones. 

Some of the stones that have been seen are not 
the true turquoise but odontolite, or bone turquoise, 
which consists of the teeth and bones of mastodon 
or other extinct animals, phosphate of iron being 
the colouring material. These stones may easily 
be recognized by their organic structure, which is 
clearly visible if viewed with a strong lens or under 
the microscope. Moreover, odontolite invariably 
contains some calcium carbonate, and effervescence 
takes place if it be touched with hydrochloric acid. 
Turquoise dissolves in hydrochloric acid, but 
without effervescence, and since it contains copper, 
a fine blue colour is imparted to the solution by 
the addition of ammonia. Odontolite has a higher 
specific gravity, 3*0 to 3*5, but lower hardness, 
5 on Mohs's scale. 

Variscite, the hydrated phosphate of aluminium, 
corresponding to the formula A1PO 4 + 2H 2 O, is found 
in masses resembling a greenish turquoise, but it 
is much softer, being only 4 on Mohs's scale. The 
specific gravity is 2'55. Round nodular masses of 
variscite are found in Utah. 



CHAPTER XXXIII 
JADE 

THOUGH not usually accounted precious 
among European nations or in Western 
civilization in general, jade was held in extraordi- 
nary esteem by primitive man, and was fashioned 
by him into ornaments and utensils, often of con- 
siderable beauty, and even at the present day it 
ranks among the Chinese and Japanese peoples 
above all precious stones ; indeed, the Chinese word 
Yu and the Japanese words Giyuku or Tama 
signify both jade and precious stones in general. 
According to the Chinese, jade is the prototype of all 
jems, and unites in itself the five cardinal virtues 
Jin, charity ; Gi, modesty ; Yu, courage ; Ketsu, 
justice ; and Chi, wisdom. When powdered and 
mixed with water, it is supposed to be a powerful 
remedy for all kinds of internal disorders, to 
strengthen the frame and prevent fatigue, to prolong 
life, and, if taken in sufficient quantity just before 
death, to prevent decomposition. 

Jade is a general term that includes properly two 
distinct mineral species, nephrite or greenstone, 
and jadeite, which are very similar in appearance, 
both being fibrous and tough in texture, and more 
or less greenish in colour ; but it is also applied 
to other species such as saussurite, californite, 



JADE 261 

bowenite, and plasma, which have somewhat similar 
characters. The word jade is a corruption of the 
Spanish pietra di hijada, kidney-stone, in allusion 
to its supposed efficacy in diseases of that organ. 

Nephrite or greenstone (Plate XXIX, Fig. 16) 
is the commoner of the two jades. It is closely 
allied to the mineral hornblende, a silicate of 
magnesium, iron, and calcium corresponding to the 
formula Ca(Mg,Fe) 3 (SiO 3 ) 4 , the magnesia being re- 
placeable by ferrous oxide. Microscopic examina- 
tion shows that the structure consists of innumerable 
independent fibres foliated or matted together, the 
former character giving rise to a slaty and the 
latter to a horny appearance in the stone as seen 
by the unaided eye. The colour varies from grey 
to leaf- and dark-green, the tint deepening as the 
relative amount of iron in the composition increases, 
and brown tints result from the oxidation of the 
iron along cracks in the stone. The hardness is 
6 \ on Mohs's scale; nephrite is therefore about as 
hard as ordinary glass and softer than quartz. 
When polished, it always acquires a greasy lustre. 
The specific gravity ranges from 2-9 to 3-1. The 
least and greatest of the principal refractive indices 
are I '606 and i'632 respectively, the double 
refraction being biaxial and negative ; the coloured 
fibres also display dichroism. All these differential 
effects are, however, masked in the stone because of 
the irregularity of the aggregation. Nephrite is 
fusible before the blowpipe, but only with difficulty. 
Its name is derived from the Greek word ve<f>po<i, 
kidney, the allusion being the same as for jade. 

Many of the prehistoric implements found in 
Mexico and in the Swiss Lake Habitations are 



262 GEM-STONES 

composed of nephrite, but it is uncertain where the 
mineral was obtained. Much of the material used 
by the Chinese at the present time comes from 
spots near the southern boundary of Eastern 
Turkestan, especially in the valleys of the rivers 
Karakash and Yarkand in the Kwen Lun range of 
mountains; it is also found farther north at the 
river Kashgar. It occurs in various provinces 
of China, namely, Shensi, Kwei Chau, Kwang Tung, 
Yunnan, and Manchuria. Gigantic waterworn 
boulders have been found in the Government of 
Irkutsk, near Lake Baikal, in eastern Siberia, the 
first discovery being made in the bed of the Onot 
stream by the explorer and prospector J. P. Alibert, 
in 1850. A large boulder of this kind, weighing 
over half a ton (1156 lb., or 524-5 kg.), is exhib- 
ited in the Mineral Gallery of the British Museum 
(Natural History). An enormous mass, weighing over 
2 tons (4718 lb., or 2140 kg.), was discovered at 
Jordansmiihl, Silesia, by Dr. G. F. Kunz, and is now 
in the magnificent collection of jade formed by 
Mr. Heber R. Bishop. Beautiful greenstone occurs 
in New Zealand, particularly in the Middle Island. 
The Maoris have long used it for various useful and 
ornamental purposes, the most common being 
indicated by their general name for the species, 
punamu, axe-stone ; kawakawa is the ordinary 
green variety, a fine section of which is shown on 
the wall of the Mineral Gallery of the British 
Museum (Natural History), while inanga, a grey 
variety, and kakurangi, a pale-green and translucent 
variety, are rare and highly prized. 

Jadeite (Plate XXIX, Fig. 18) is by far the rarer 
of the two jades, and is the choicest gem with the 



JADE 263 

Chinese. In composition it is a silicate of sodium 
and aluminium with the formula NaAl(SiO 3 ) 2 , corre- 
sponding to the lithium mineral spodumene (p. 265). 
It has the same toughness and greasy lustre as 
nephrite, but is harder, being represented by the 
symbol 7 on Mohs's scale, and thus only slightly, 
if at all, softer than quartz. The other characters 
are also higher; the specific gravity is about 3^34, 
and the least and greatest of the principal refractive 
indices are i'66 and r68, the double refraction 
being biaxial and negative. The colour varies 
from white to almost an emerald green, the latter 
being especially prized, and often the green colour 
runs in streaks through the white. Jadeite fuses 
readily before the blowpipe to blebby glass, more 
easily than is the case with nephrite. 

The finest jadeite comes from the Mogaung 
district in Upper Burma, where it is found in 
boulders and also with albite in dykes in a dark- 
green serpentine. The export trade to China, which 
absorbs practically the whole of the output, is 
exceedingly valuable, and realizes nearly as much 
as the produce of the ruby mines. Jadeite is also 
found in the Shensi and Yunnan provinces of China, 
and in Tibet. 

A few words may be said about the other jade- 
like minerals. Saussurite, which is named after 
H. B. de Saussure, has resulted from the decomposi- 
tion of a felspar, and is nearly akin to the mineral 
zoisite. It has the customary toughness of structure, 
and is greenish grey to white in colour. Its specific 
gravity is about 3 '2, and hardness 6 to 7 on Mohs's 
scale. It occurs near Lake Geneva. Bowenite is 



264 GEM-STONES 

a green serpentine (p. 289) which is found at 
Smithfield, Rhode Island, U.S.A., and in New 
Zealand and Afghanistan. Californite and plasma 
are compact varieties of idocrase (p. 275) and 
chalcedony (p. 247) respectively. Verdite is a stone 
of rich green colour which is found in the form of 
large boulders in the North Kaap River, South 
Africa ; it is composed of green mica (fuchsite) and 
some clayey matter. 

Jade has of recent years been imitated in glass, 
but the latter is recognizable by its vitreous lustre 
and inferior hardness, and sooner or later by its 
frangibility. 



CHAPTER XXXIV 
SPODUMENE, IOLITE, BENITOITE 

SPODUMENE 
(Kunzite, Hiddenite) 

TILL a few years ago scarcely known out- 
side the ranks of mineralogists, spodumene 
suddenly leaped into notice in 1903 upon the 
discovery of the lovely lilac-coloured stones (Plate 
XXIX, Fig. 10) at Pala, San Diego County, Cali- 
fornia; they shortly afterwards received the name 
kunzite after the well-known expert in gems, Dr. 
G. F. Kunz. The stones were found here in a peg- 
matite dyke, and were of all shades, ranging from 
pale pink to deep lilac, and at times as much as 
150 carats in weight. Paler kunzite occurs with 
beryl and tourmaline at Coahuila Mountain in River- 
side County, California, and colourless stones have 
recently come to light in Madagascar. Kunzite 
is remarkable for its wonderful dichroism ; the 
beautiful violet tint that springs out in one direction 
comes with greater surprise because of the un- 
interesting yellowish tints in other directions. 
Unlike spodumene in general, kunzite is phosphor- 
escent under the influence of radium. 

The emerald-green variety (Plate XXIX, Fig. 1 1), 



266 GEM-STONES 

named hiddenite after Mr. W. E. Hidden, who 
discovered in 1881 the only known occurrence, in 
Alexander County, North Carolina, would no doubt 
have become popular had the supply of material not 
been so very limited ; few stones were found, and 
the variety has never come to light elsewhere. The 
colour is supposed to be due to chromic acid. 
Hiddenite being also dichroic, the tint varies with 
the direction. . 

Spodumene is ordinarily rather a pale yellowish 
in hue, and, as its name (which is derived from 
<77ro8('o9, ash-coloured) suggests, is not very attractive. 
Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are 
found in Brazil and Madagascar. 

The species is interesting scientifically because it 
contains the rare element lithium ; it is a silicate of 
aluminium and lithium, corresponding to the formula 
LiAl(SiO 3 ) 2 . The double refraction is biaxial \m 
character and positive in sign, the least and greatest 
of the refractive indices being r66o and 1*675 > the 
specific gravity is S'iS'S, and hardness 6 to 7 on 
Mohs's scale. Spodumene has an easy cleavage, 
and the cut stones call therefore for careful handling, 
lest they be flawed or fractured. Two faceted 
stones, a beautiful kunzite and a fine hiddenite, 
weighing 60 and 2.\ carats respectively, are ex- 
hibited in the British Museum (Natural History). 

lOLITE 

Known also by various other names cordierite, 
dichroite, and water-sapphire (saphire <?eau*) this 
species owes its interest to the remarkable dichroism 
characterizing it, the principal colours smoky-blue 



SPODUMENE, IOLITE, BENITOITE 267 

and yellowish white being in such contrast as to 
be obvious to the unaided eye. The stones that 
are usually worked have intrinsically a smoky-blue 
colour, and are found in watenvorn masses in the 
river-gravels of Ceylon, whence is the origin of the 
name water-sapphire. lolite, from LOV, violet, and 
Xi#o9, stone, refers to the colour ; cordierite is named 
after Cordier, a French geologist, who first studied 
the crystallography of the species ; and dichroite, of 
course, alludes to the most prominent character of 
the species. 

lolite is a silicate of aluminium and of magnesium 
and iron corresponding to the formula H 2 (Mg,Fe) 4 
Al 8 Si 10 O 37 . The double refraction is small in 
amount, biaxial in character, and negative in sign, 
the least and greatest of the refractive indices being 
1-543 an d 1*55 *J th 6 specific gravity is 2*63, and 
hardness 7 on Mohs's scale. lolite, if used, is 
worked and polished; it is seldom faceted. A 
large worked piece, weighing 177 grams, which was 
formerly in the Hawkins Collection, is exhibited in 
the British Museum (Natural History). 

BENITOITE 

The babe among gem-stones, benitoite first saw 
the light of day a few years ago, early in 1907. 
It occurs with the rare mineral neptunite, which was 
previously known only from Greenland, in narrow 
veins of natrolite in Diablo Range near the head- 
waters of the San Benito River, San Benito County, 
California. Despite careful search the species has 
not been found except within the original restricted 
area. To science it is interesting both because of 



268 GEM-STONES 

its composition, a silico-titanate of barium, corre- 
sponding to the formula BaTiSi 3 O 9 , and because its 
crystals belong to a class of crystalline symmetry 
which has hitherto not been represented among 
minerals. The double refraction is uniaxial, and 
since the ordinary index of refraction is 1757 and 
the extraordinary 1-804, it is positive in sign and 
large in amount, namely, O'O47. The stones are 
characterized by strong dichroism, the colour corre- 
sponding to the ordinary ray being white, and to the 
extraordinary greenish blue to indigo depending 
upon the tint of the stone. To obtain the best 
effect the stone must therefore be cut with the table- 
facet parallel to the crystallographic axis. The 
specific gravity is 3'65, and hardness 6| on Mohs's 
scale. When first discovered the species was 
supposed to be sapphire, and many stones were cut 
and sold as such. It is, however, much softer than 
sapphire, and is readily distinguished by its optical 
characters, since it possesses greater double refraction 
and of differing sign, so that, when tested with the 
refractometer, the shadow-edge corresponding to the 
lower index of refraction remains fixed in the case of 
of benitoite, whereas the contrary happens with 
sapphire. Benitoite also, unlike sapphire, fuses 
easily to a transparent glass. Its blue colour, 
which is supposed to be due to a small amount of 
free titanic acid present, appears to be stable. 
Several stones as large as I to 2 carats in weight 
have been found. The largest of all, perfectly flaw- 
less, weighs just over 7 carats, and is remarkable 
because it is about three times the next largest in 
point of weight ; it is the property of Mr. G. Eacret, 
of San Francisco. 



CHAPTER XXXV 
EUCLASE, PHENAKITE, BEItYLLONITE 

EUCLASE 

THIS species comes near beryl in chemical com- 
position, being a silicate of aluminium and 
beryllium corresponding to the formula Be(AlOH) 
SiO 4 , and closely resembles aquamarine in colour 
and appearance when cut. Owing to the rarity of 
the mineral good specimens command high prices 
for museum collections, and it is seldom worth while 
cutting it for jewellery. It derives its name from its 
easy cleavage, tv easily, and /eXatri? fracture. The 
double refraction is biaxial in character and positive 
in sign, the least and greatest of the refractive 
indices being 1-651 and 1-670 respectively; the 
specific gravity is 3*07, and the hardness 7| on 
Mohs's scale. The colour is usually a sea-green, 
but sometimes blue. Euclase occurs with topaz at 
the rich mineral district of Minas Novas, Minas 
Geraes, Brazil, and has also been found in the Ural 
district, Russia. 

PHENAKITE 

Another beryllium mineral, phenakite owes its 
name to the frequency with which it has been 

mistaken for quartz, being derived from </>tWf, 
269 



270 GEM-STONES 

deceiver. The clear, colourless crystals, somewhat 
complex in form, have at times been cut, but they 
lack 'fire,' and despite their brilliant lustre meet 
with little demand. The composition is a silicate of 
beryllium corresponding to the formula Be 2 SiO 4 . 
The double refraction is uniaxial, and since the 
ordinary, 1*652, is less than the extraordinary index, 
1-667, it is positive in sign; the specific gravity is 
2'99, and the hardness is almost equal to that of 
topaz, being about 7 to 8 on Mohs's scale. 

Fine stones have long been known near Ekaterin- 
burg in the Ural Mountains, and have recently been 
discovered in Brazil. 



BERYLLONITE 

As its name suggests, this mineral also contains 
beryllium, being a soda phosphate corresponding to 
the formula NaBePO 4 . Clear, colourless stones, 
which occur at Stoneham, Maine, U.S.A., have been 
cut, but the lack of ' fire,' the easy cleavage, and 
comparative softness, the symbol being 5| on Mohs's 
scale, unfit it for use in jewellery. The double re- 
fraction is biaxial in character and negative in sign, 
the least and the greatest of the refractive indices 
being i'553 and 1-565 respectively. 



CHAPTER XXXVI 

ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, 
IDOCRASE, EPIDOTE, SPHENE, AXINITE, 
PREHNITE, APATITE, DIOPTASE 

ENSTATITE 
(' Green Garnet ') 

THE small green stones which accompany 
the diamond in South Africa have been cut 
and put on the market as ' green garnet.' They 
are, however, in no way connected with garnet, but 
belong to a mineral species called enstatite, which is 
a silicate of magnesium corresponding to the formula 
MgSiO 3 ; the green colour is due to a small amount 
of ferrous oxide which replaces magnesia. The 
double refraction is biaxial in character and positive 
in sign, the least and greatest of the refractive 
indices being 1*665 an d 1*674 respectively; the 
specific gravity ranges from 3*10 to 3*13, and the 
hardness is only about S| on Mohs's scale. The 
dichroism is perceptible, the twin-colours being 
yellowish and green, and, as usual, is more pro- 
nounced the deeper the colour of the stone. There 
is also a good cleavage in two different directions. 

With increasing percentage amount of iron 
enstatite passes into hypersthene. The colour 



272 GEM-STONES 

becomes a dark brownish green, and an increase 
takes place in the physical constants, the least and 
greatest of the refractive indices attaining to 1*692 
and i '7 5 respectively, and the specific gravity 
ranging from 3*4 to 3*5. Hypersthene is never 
sufficiently transparent for faceting, but when 
spangled with small scales of brookite it is sometimes 
cut en cabochon. 

The name enstatite is derived from ei/o-Tar^<?, an 
opponent, referring to the infusibility of the mineral 
before the blowpipe, and hypersthene comes from 
t>7re/3<7#ei/o9, very tough. 

An altered enstatite, leek-green in colour and 
with nearly the composition of serpentine (p. 289), 
has been cut en cabochon. It has much lower 
specific gravity, only 2*6, and lower hardness, 3| to 
4 on Mohs's scale. It is named bastite from Baste 
in the Harz Mountains, where it was first discovered. 

DlOPSlDE 

This species, which is also known as malacolite 
and alalite, provides stones of a leaf-green colour 
which have occasionally been cut. It is a silicate 
of calcium and magnesium corresponding to the 
formula MgCa(SiO 3 ) 2 , but usually contains in place 
of magnesia some ferrous oxide, to which it owes its 
colour; with increase in the percentage amount of 
iron the colour deepens and the physical constants 
change. The double refraction is large in amount, 
0*028, biaxial in character, and positive in sign. 
The least and greatest of the refractive indices 
corresponding to the stones suitable for jewellery 
range about 1-671 and 1*699 respectively, but they 



KYANITE 273 

may be as high as 1732 and 1750 in the two 
cases. The specific gravity varies from 3*20 to 3 '3 8, 
and the hardness from 5 to 6 on Mohs's scale. 
Dichroism is noticeable in deep-coloured stones, but 
is not very marked. 

The name diopside comes from 19, double, and 
0^9, appearance, in allusion to the effect resulting 
from the double refraction ; malacolite is derived 
from /LiaXa#o9, soft, because the mineral is softer than 
the felspar associated with it ; and alalite is named 
after the principal locality, Ala Valley, Piedmont, 
Italy. 

KYANITE 

Kyanite, also known as disthene, is interesting for 
two reasons. Its structure is so grained in character 
that the hardness varies in the same stone from 5 to 
7 on Mohs's scale ; it can therefore be scratched by 
a knife in some directions, but not in others (p, 79). 
It has the same chemical composition as andalusite, 
both being silicates of aluminium corresponding to 
the formula Al 2 SiO 6 , but possesses very different 
physical characters, a fact which shows how large a 
share the molecular grouping has in determining the 
aspect of crystallized substances. It is biaxial with 
small negative double refraction, the least and 
greatest of the refractive indices being 172 and 
173 respectively; the specific gravity is 3*61. It 
occurs in sky-blue prismatic crystals, whitish at the 
edges, in schist near St. Gothard, Switzerland. It is 
seldom cut. 

Kyanite is derived from its colour, tcvavos blue, 
and disthene, from its variable hardness, Bl<s, twice, 
and aOevos, strong. 
18 



274 GEM-STONES 

ANDALUSITE 

Andalusite bears no resemblance whatever to 
kyanite, although, as has been stated above, the 
composition of the two species is essentially the 
same. It is usually light bottle-green in colour, 
and more rarely brown and reddish. Its extreme 
dichroism is its most remarkable character, the twin 
colours being olive-green and red. The reddish 
gleams that are reflected from the interior are in 
sharp contrast with the general colour of the stone, 
and impart to it a weird effect (Plate XXIX, Fig. 1 5). 
Cut stones are often confused with tourmalines, and 
can, indeed, only be distinguished from the latter 
with certainty by noting on the refractometer the 
smaller amount of double refraction and the differ- 
ence in its character. The least and greatest of the 
refractive indices are 1*632 and r643 respectively, 
and the double refraction, O'Oi I, about half that of 
tourmaline, is biaxial and negative; the specific 
gravity is 3*18, and hardness 7^ on Mohs's scale. 

Good stones are found at Minas Novas, Minas 
Geraes, Brazil, and in the gem-gravels of Ceylon. 
It was first known from the province of Andalusia, 
Spain, whence is the origin of its name. 

IDOCRASE 
( Vesuvianite, Calif ornite) 

Idocrase, also known as vesuvianite, is occasionally 
found in the form of transparent, leaf-green, and 
yellowish-brown stones which, when cut, may be 
mistaken for diopside and epidote respectively, but 
are distinguishable from both by the extreme small- 



EPIDOTE 275 

ness of their double refraction. Californite is a 
compact variety which has all the appearances of a 
jade; its colour is green, or nearly colourless with 
green streaks. 

In composition idocrase is a silicate of aluminium 
and calcium, the precise formula of which is un- 
certain, but may be 

(Ca ) Mn,Mg,Fe) 2 [(Al,Fe)(OH > F)]Si 2 7 . 

The double refraction, which is uniaxial in character 
and negative in sign, may be less than crooi, and 
never exceeds o - oo6, so that it is not easily detected 
with the refractometer, even in sodium light The 
refractive indices vary enormously in value, from 
1*702 to I "j 26 for the ordinary, and from 1-706 
to 1732 for the extraordinary ray. The specific 
gravity varies from 3*35 to 3*45, and the hardness 
is about 6^ on Mohs's scale. 

The name idocrase, from etSo?, form, and icpcUris, 
mixture, was assigned to the species by Haiiy, 
but his reasons have little meaning at the present 
day. The other names are taken from the localities 
where the species and the variety were first discovered. 

Bright, green crystals come from Russia, and 
also from Ala Valley, Piedmont, and Mount Vesu- 
vius, Italy. Californite is found in large masses in 
Siskiyon and Fresno Counties, California. 

EPIDOTE 
(Pistactte) 

Epidote often possesses a peculiar shade of 
yellowish green, similar to that of the pistachio-nut 
hence the origin of its alternative name which is 



276 GEM-STONES 

unique among minerals, though scarcely pleasing 
enough to recommend it to general taste. Its ready 
cleavage renders it liable to flaws; nevertheless, it 
is occasionally faceted. The name epidote, from 
eVtSoo-i?, increase, was given to it by Haiiy, but not 
on very precise crystallographical grounds. 

In composition this species is a silicate of calcium 
and aluminium, with some ferric oxide in place of 
alumina, corresponding to the complex formula, 
Ca2(Al,Fe) 2 [(Al,Fe)OH](SiO 4 ) 3 . It occurs in mono- 
clinic, prismatic crystals richly endowed with 
natural faces. The colour deepens with increase 
in the percentage amount of iron, and the stones 
become almost opaque. The double refraction is 
large in amount, 0*031, biaxial in character, and 
negative in sign. The dichroism is conspicuous in 
transparent stones, the twin-tints corresponding to 
the principal optical directions being green, brown, 
and yellow. The values of the least and greatest 
of the refractive indices given by transparent stones 
are 1*735 and 1*766 respectively; the specific 
gravity varies from 3*25 to 3*50, and the hardness 
from 6 to 7 on Mohs's scale. 

Transparent crystals have come from Knappen- 
wand, Untersulzbachtal, Salzburg, Austria; Traver- 
sella, Piedmont, Italy ; and Arendal, Nedenas, 
Norway. Magnificent, but very dark, crystals were 
discovered about ten years ago on Prince of Wales 
Island, Alaska. 

SPHENE 
(Titanite) 

The clear, green, yellow, or brownish stones 
provided by this species would be welcomed, in 



SPHENE 277 

jewellery because of their brilliant and almost 
adamantine lustre, but, unfortunately, they are too 
soft to withstand much wear, the hardness being 
only 5i on Mohs's scale. In composition sphene 
is a silico-titanate of calcium corresponding to the 
formula CaTiSiO 6 , and in this respect comes near 
the recently discovered gem-stone, benitoite. The 
refractive indices lie outside the range of the re- 
fractometer, the values of the least and the greatest 
of the refractive indices varying from i'888 and 
i '9 1 7 to i '9 1 4 and 2*053 respectively. It is to 
this high refraction that it owes its brilliant lustre. 
The double refraction, which is biaxial in character 
and positive in sign, is so large that the apparent 
doubling of the opposite edges of a cut stone when 
viewed through one of the faces is obvious to the 
unaided eye (cf. p. 41). Cut stones have ad- 
ditional interest on account of the vivid dichroism 
displayed, the twin-tints, colourless, yellow, and 
reddish yellow, corresponding to the three principal 
optical directions, being in strong contrast. The 
specific gravity ranges from 3*35 to 3*45. The 
negative test with the refractometer (cf. p. 26), the 
softness, and the large amount of double refraction 
suffice to distinguish this species from gem-stones 
of similar appearance. 

The name sphene, from a-Qijv, wedge, alludes to 
the shape of the natural crystals. The alternative 
name is obviously due to the fact that the species 
contains titanium. 

Good stones have come from the St. Gothard 
district, Switzerland. 



278 GEM-STONES 



AXINITE 

Called axinite from the shape of its crystals 
], axe this species supplies small, clear, clove- 
brown, honey-yellow, and violet stones which can 
be cut for those who care for a stone out of the 
ordinary. The composition is a boro-silicate of 
aluminium and calcium, with varying amounts of 
iron and manganese, corresponding to the formula 
(Ca,Fe) 3 Al 2 (B.OH)Si 4 O 15 . Axinite is interesting on 
account of its strong dichroism, the twin-tints corre- 
sponding to the principal optical directions being 
violet, brown, and green. The double refraction is 
biaxial in character and negative in sign, the least 
and greatest of the refractive indices being 1*674 
and 1*684; the specific gravity is 3'2 8, and hard- 
ness about 6-g- to 7, or rather under that of quartz. 

The best examples have been found at St. 
Cristophe, Bourg d'Oisans, in the Dauphind, France. 
Violet axinite is a novelty that has come within 
recent years from Rosebery, Montagu County, 
Tasmania. 

PREHNITE 

This species, which is named after its discoverer, 
Colonel Prehn, is found in nodular, yellow and 
oil-green stones, of which the latter have very 
occasionally been cut. It is a little soft, the 
hardness being only 6 on Mohs's scale. The 
double refraction is large in amount, 0*03 3, biaxial 
in character, and positive in sign, the least and the 
greatest of the refractive indices being I '6 1 6 and 
r649 respectively; the specific gravity varies 
from 2'8i to 2-95. In composition prehnite is a 



APATITE 279 

silicate of aluminium and calcium corresponding 
to the formula H 2 Ca 2 Al 2 (SiO 4 ) 3 . 

The best material has been found at St. 
Cristophe, Bourg d'Oisans, Dauphine", France. 



APATITE 

This interesting mineral is found occasionally 
in attractive green, blue, or violet stones, but is 
unfortunately too soft for extensive use in jewellery, 
the hardness being only 5 on Mohs's scale. In 
composition it is a fluo - chloro - phosphate of 
calcium, corresponding to the formula Ca 4 [Ca(F,Cl)] 
(PO 4 ) 3 . When pure, it is devoid of colour, the 
tints being due to the presence of small amounts 
of tinctorial agents. The double refraction is 
uniaxial in character and negative in sign, the 
ordinary index being r642 and the extraordinary 
1*646; the specific gravity varies from 3 - i7 to 
3*23. The dichroism is usually feeble, but some- 
times is strong ; for instance, in the stones from 
the Burma ruby mines (yellow, blue-green). A 
cut stone might be mistaken for tourmaline, but 
is distinguished by its softness, or, when tested on 
the refractometer, by its inferior double refraction. 
It received its name from a-jrardeiv, deceive, because 
it was wrongly assigned to at least half a dozen 
different species in early days. Moroxite is a name 
sometimes given to blue-green apatite. 

Beautiful violet stones are found at Ehrenfried- 
ersdorf, Saxony; Schlaggenwald, Bohemia; and 
Mount Apatite, Auburn, Androscoggin County, 
Maine, U.S.A. ; and blue stones come from Ceylon. 



280 GEM-STONES 

DlOPTASE 

Though of a pretty, emerald-green colour, dioptase 
has never been found in large enough crystals for 
gem purposes, and it is, moreover, rather soft, the 
hardness being only 5 on Mohs's scale, and has 
an easy cleavage. In composition it is a hydrous 
silicate of copper corresponding to the formula 
CuH 2 SiO 4 . The double refraction, which is 
large in amount, is uniaxial in character, and 
positive in sign, the ordinary refractive index 
being r66/ and the extraordinary i"J2^. Its 
colour and softness distinguish it from peridot or 
diopside, which have about the same refractivity. 
The name was assigned to the species by HaUy, 
from Bia, through, and oTrro/Aat, see, because the 
cleavage directions were distinguishable by looking 
through the stone. 

Dioptase has been found near Altyn-Ttibe in 
the Kirghese Steppes, at Rezbanya in Hungary, 
and Copiapo in Chili, and at the mine Mindouli, 
near Comba, in the French Congo. 



CHAPTER XXXVII 
CASSITERITE, ANATASE, PYRITES, HEMATITE 

CASSITERITE 

THOUGH usually opaque, this oxide of tin, 
corresponding to the formula SnO 2 , has 
occasionally, but very rarely, been found in small, 
transparent, yellow and reddish stones suitable 
for cutting. The lustre is adamantine. The 
refraction is uniaxial in character and positive in 
sign, the ordinary index being 1*997 and extra- 
ordinary 2-093. The specific gravity is high, 
ranging from 6'8 to yi. The hardness is on the 
whole less than that of quartz, being about 6 to 7 
on Mohs's scale. 

ANATASE 

This mineral, which is one of the three crys- 
tallized forms of titanium oxide, TiO 2 , occurs 
often in small, brown, transparent stones which 
occasionally find their way into the market. The 
lustre is adamantine. The refraction is uniaxial 
in character and negative in sign, the extraordinary 
index being 2-493 and ordinary 2-554. The 
specific gravity varies from 3-82 to 3-95, and the 
hardness is about 5 i to 6 on Mohs's scale. 



282 GEM-STONES 

PYRITES, HEMATITE 

These two metallic minerals were employed in 
ancient jewellery. The former, sulphide of iron, 
FeS 2 , is brass-yellow in colour, and has a specific 
gravity 5*2, and hardness 6\ on Mohs's scale. It 
is found, when fresh, in brilliant cubes. The latter, 
oxide of iron, Fe 2 O 3 , has a black metallic lustre, 
but, when powdered, is red in colour a mode of 
distinguishing it from other minerals of similar 
appearance. Its specific gravity is 5^3, and hard- 
ness 6 on Mohs's scale. In modern times it has 
been cut in spherical form to imitate black pearls, 
but can easily be recognized by its greater density 
and hardness. Hematite is used for signet stones, 
often with an intaglio engraving. 



CHAPTER XXXVIII 
OBSIDIAN, MOLDAVITE 

TWO forms of natural glass have been em- 
ployed for ornamental purposes. Obsidian 
results from the solidification without crystallization 
of lava, and corresponds in composition to a granite. 
The structure is seldom clear and transparent, and 
usually contains inclusions or streaks. The colour 
is in the mass jet-black, but smoky in thin frag- 
ments, and occasionally greenish. Its property of 
breaking with a keen cutting edge, in the same 
way as ordinary glass, rendered it of extreme 
utility to primitive man, who was ignorant of the 
artificial substance. The refraction is, of course, 
single, and the refractive index approximates to 
1-50. The specific gravity varies from 2*3 to 2-5. 
The hardness is 5 on Mohs's scale, the same as 
ordinary glass. 

Obsidian is obtained wherever there has been 
volcanic activity. Vast mines of great antiquity 
exist in the State of Hidalgo, Mexico. 

Moldavite, which differs in no respect from 
ordinary green bottle-glass, is of interest on account 
of its problematical origin. Its occurrence in 
various parts of Bohemia and Moravia cannot be 
explained as the result of volcanic agency. It 
may possibly be the product of old and forgotten 

a8 3 



284 GEM-STONES 

glass factories which at one time existed on the 
site. Even meteorites have been suggested as 
the source. The physical characters are the same 
as those of ordinary glass : refraction single, index 
1*51; specific gravity 2-50 and hardness 5-5- on 
Mohs's scale. Moldavite also passes under the 
names of bottle-stone, or water - chrysolite. A 
natural glass of the same character has been found 
in water-worn fragments in Ceylon, and has been 
sold as peridot, which it resembles in colour, but is 
readily distinguished from it by its very different 
physical properties. 



PART II SECTION C 
ORNAMENTAL STONES 

CHAPTER XXXIX 

FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, 
RHODONITE, AZURITE, MALACHITE, 
THULITE, MARBLE, APOPHYLLITE, CHRY- 
SOCOLLA, STEATITE OR SOAPSTONE, 
MEERSCHAUM, SERPENTINE 

SPACE will not permit of more than a few 
words concerning the more prominent of the 
numerous mineral species which are employed for 
ornamental purposes in articles of virtu or in archi- 
tecture, but which for various reasons cannot take 
rank as gem-stones. 

Fluor, a beautiful mineral which is found in its 
greatest perfection in England, has enjoyed well- 
deserved popularity when worked into vases or other 
articles. The finest material, deep purple in colour, 
known as ' Blue John/ came from Derbyshire, but 
the supply is now exhausted. The crystallized 
examples, from Durham, Devonshire, and Cornwall, 
form some of the most attractive of museum 
specimens. The crystals take the shape of cubes, 
often twinned, and have an easy octahedral cleavage. 



286 GEM-STONES 

The refraction is single, the index being i'433. 
Fluor is noted for its property of appearing of 
differing colour by reflected and transmitted light, 
and the phenomenon is in consequence known as 
fluorescence. The specific gravity is 3*18, and the 
hardness 4 on Mohs's scale. Owing to its low 
refraction and softness, fluor is not suitable for 
jewellery. Clear colourless material is in demand 
for particular lenses of microscope objectives. 

The lovely blue stone known as lapis lazuli has 
since the earliest times been applied to all kinds of 
decorative purposes, for mosaic and inlaid work and 
as the material for vases, boxes, and so on, and was 
the original sapphire of the ancients. When ground 
to powder it furnishes a fine blue paint, but it has 
now been entirely superseded for this purpose by an 
artificial product. Although to the eye so homo- 
geneous and uniform in structure, lapis lazuli has 
been shown by microscopic examination to be 
composed of calcite coloured by three blue minerals 
in varying proportions. All three belong to the 
cubic class of symmetry, and are mainly soda 
aluminium silicates in composition ; their hardness 
varies from 5 to 6 on Mohs's scale. Lazurite, 
Na 4 (NaS 3 .Al)Al 2 Si 3 O 12 , has specific gravity varying 
from 2*38 to 2'45, and hardness about 5 to 5^; 
hauynite, (Na 2 ,Ca) 2 (NaSO 4 ,Al)Al 2 Si 3 O 12 , is about 
the same in specific gravity, 2*4 to 2'5, but slightly 
harder, 5 to 6 ; while sodalite, Na 4 (AlCl)Al 2 Si 3 O 12 , 
is the lightest in density, 2-14 to 2-30, with hardness 
5 to 6, and has a refractive index I '4 8 3. 

By far the oldest mines are in the Badakshan 
district of Afghanistan, a few miles above Firgamu 
in the valley of the Kokcha, a branch of the Oxus, 



SODALITE, VIOLANE, RHODONITE 287 

where ruby and spinel are found. It is also 
found at the southern end of Lake Baikal, Siberia, 
and in the Chilian Andes. 

Sodalite occurs in beautiful blue masses at 
Dungannon, Hastings County, Ontario, Canada, 
and at Litchfield, Maine, U.S.A. They make 
excellent polished stones. 

Violane, a massive, dark violet-blue diopside from 
San Marcel, Piedmont, Italy, also makes a handsome 
polished stone. 

Rhodonite, silicate of manganese, MnSiO 3 , 
possesses a fine red colour, and makes an attractive 
stone when cut and polished. It has very slight 
biaxial double refraction, the refractivity being about 
173 ; the specific gravity is 3-6, and hardness 6. 
It is found in large masses near Ekaterinburg in the 
Ural Mountains, and is quarried as an ornamental 
stone. 

Both the copper carbonates, azurite or chessylite, 
and malachite, make effective polished stones. The 
latter is also worked into various ornamental objects ; 
it occurs in fibrous masses, the grained character of 
which look well in the polished section. Its colour 
is a bright green, to which it owes its name, from 
fj,a\a,Kr), mallows. Its composition is represented by 
the- formula CuCO 3 .Cu(OH) 2 , and it is the more 
stable form, since azurite is frequently found altered 
to it. It has biaxial double refraction, and the 
indices are about r88 ; the specific gravity is 4*01, 
and hardness about 3^ to 4 on Mohs's scale. It is 
found in large masses at the copper mines of Nizhni 
Tagilsk in the Ural Mountains, where it is mined as 
an ornamental stone ; it also accompanies the copper 
ores in many parts of the world, for instance Cuba, 



288 GEM-STONES 

Chili, and Australia. Azurite, so called on account 
of its beautiful blue colour, is rarer, but, unlike 
malachite, is generally in the form of crystals. 
Beautiful specimens have come from Chessy, near 
Lyons, France, and Bisbee, Arizona, U.S.A. The 
composition corresponds to the formula 2CuCO 3 , 
Cu(OH) 2 . The specific gravity is 3'8o, and hard- 
ness about 3 1 to 4. 

Chrysocolla occurs in blue and bluish-green 
earthy masses, with an enamel-like texture, which in 
some instances can be worked and polished. Being 
the result of the decomposition of copper ores, it 
varies considerably in hardness, ranging from 2 to 
4 on Mohs's scale. Its composition approaches to 
the formula CuSiO 3 .2H 2 O, but it invariably contains 
impurities. It is very light, the density being only 
about 2-2. 

Steatite, or soapstone, is a massive foliated sili- 
cate of magnesium corresponding to the formula 
H 2 Mg 3 Si 4 O 12) which is one of the softest of mineral 
substances, representing the degree I on Mohs's 
scale, but in massive pieces is harder owing to the 
intermixture of other substances with it. It has a 
peculiar greasy feeling to the touch, due to its softness. 
The specific gravity is about 275. The Chinese carve 
images out of the yellowish and brownish pieces. 

Meerschaum, a silicate of magnesium corre- 
sponding to the formula H 4 Mg 2 Si 3 O 10 , is familiar 
to every smoker as a material for pipe-bowls. It 
is very light, the specific gravity being only 2'O, 
and soft, the hardness being about 2 to 2 on Mohs's 
scale. When found, it is pure white in colour, and 
answers to its name, a German word signifying sea~ 
foam, It comes from Asia Minor. 



SERPENTINE 289 

Serpentine has been largely used for decorative 
purposes, as well as for cameos and intaglios, and 
formed most of the famous ' verde antique.' Being 
the result of the decomposition of other silicates it 
varies enormously in appearance and characters, but 
the most attractive stones are a rich oil-green in 
colour and resemble jade. The composition approxi- 
mates to the formula H 4 Mg 3 Si 2 O 9 , but it invariably 
contains other elements. The hardness varies from 
2\ to 4 on Mohs's scale, according to the minerals 
contained in the stone ; the specific gravity is about 
2 '60 and the refractivity i'57O. 

The beautiful rose-red stone, thulite, makes a 
handsome decorative stone. It has nearly the same 
composition as epidote (p. 275), and like it has 
strong dichroism, the principal colours being yellow, 
light rose, and deep rose. The colour is due to 
manganese. Its refractive index is about i'7o, 
specific gravity 3' 12, and hardness 6 to 6 on 
Mohs's scale ; it possesses an easy cleavage. Fine 
specimens come from Telemark, Norway, and it is 
therefore called after the old name for Norway, 
Thule. 

Marble is a massive calcite, carbonate of lime, 
with the formula CaCO 3 . When pure it is white, 
but it is usually streaked with other substances 
which impart a pleasing variety to its appearance. 
It is always readily recognized by the immediate 
effervescence set up when touched with a drop of 
acid. Calcite is highly doubly refractive (cf. p. 40), 
the extraordinary index being 1-486, and ordinary 
1-658, a difference of 0*172 ; the specific gravity is 
2*71, and hardness 3 on Mohs's scale. Lumachelle, 
or fire-marble, is a limestone containing shells from 
19 



290 GEM-STONES 

which a brilliant, fire-like chatoyancy is emitted 
when light is reflected at the proper angle. It 
sometimes resembles opal-matrix, but is easily dis- 
tinguished by its lower hardness and by its effer- 
vescent action with acid. Choice specimens come 
from Bleiberg in Carinthia, and from Astrakhan. 

Apophyllite has not many characters to commend 
it, being at the best faintly pinkish in colour, and 
always imperfectly transparent. It is a hydrous 
silicate of potassium and calcium with the complex 
formula (H,K) 2 Ca(SiO 3 ) 2 .H 2 O. Its refractivity is 
about 1*535, specific gravity 2*5, and hardness 4| on 
Mohs's scale ; it possesses an easy cleavage. It 
occurs in the form of tetragonal crystals at Andreas- 
berg in the Harz Mountains, and in the Syhadree 
Mountains, Bombay, India 



PART II SECTION D 
ORGANIC PRODUCTS 

CHAPTER XL 
PEARL, CORAL, AMBER 

A LTHOUGH none of the substances considered 
A\ in this chapter come within the strict defini- 
tion of a stone, since they are directly the result of 
living agency, yet pearl at least cannot be denied 
the title of a gem. Both pearl and coral contain 
calcium carbonate in one or other of its crystallized 
forms, and both are gathered from the sea ; but 
otherwise they have nothing in common. Amber 
is of vegetable origin, and is a very different 
substance. 

PEARL 

From that unrecorded day when some scantily 
clothed savage seeking for succulent food opened an 
oyster and found to his astonishment within its shell 
a delicate silvery pellet that shimmered in the light 
of a tropical sun, down to the present day, without 
intermission, pearl has held a place all its own in the 
rank of jewels. Though it be lacking in durability, 
its beauty cannot be disputed, and large examples, 



292 GEM-STONES 

perfect in form and lustre, are sufficiently rare to 
tax the deepest purse. 

The substance composing the pearl is identical 
with the iridescent lining mother-o'-pearl or nacre, 
as it is termed of the shell. Tortured by the 
intrusion of some living thing, a boring parasite, 
a worm, or a small fish, or of a grain of sand or 
other inorganic substance, and without means to free 
itself, the mollusc perforce neutralizes the irritant 
matter by converting it into an object of beauty 
that eventually finds its way into some jewellery 
cabinet. Built up in a haphazard manner and not 
confined by the inexorable laws of intermolecular 
action, a pearl may assume any and every variety 
of shape from the regular to the fantastic. It may 
be truly spherical, egg- or pear-shaped pear-drops 
or pear-eyes, as they are termed or it may be 
quite irregular the so-called baroque or barrok 
pearls. The first is the most prized, but a well- 
shaped drop-pearl is in great demand for pendants 
or ear-rings. The colour is ordinarily white, or 
faintly tinged yellowish or bluish, and somewhat 
rarely, salmon-pink, reddish, or blackish grey. 
Perfect black pearls are valuable, but not as costly 
as th&. finest of the white. Though not transparent, 
pearl is to a varying extent translucent, and its 
characteristic lustre ' orient ' in the language of 
jewellery is due to the same kind of interaction of 
light reflected from different layers that has been 
remarked upon in the case of opal and certain other 
stones. The translucency varies in degree, and some 
jewellers speak of the ' water ' of pearls just as in 
the case of diamonds. If a pearl be sliced across 
the middle and the section be examined under the 



PEARL, CORAL, AMBER 293 

microscope, it will be seen that the structure consists 
of concentric shells and resembles that of an onion. 
These shells are alternately composed of calcium 
carbonate in its crystallized form, aragonite, and of 
a horny organic matter known as conchiolin, and 
they evidently represent the result of intermittent 
growth. Because of their composite character, 
pearls have a specific gravity ranging from 2-65 
to 2^69 2*84 2-89 in the case of pink pearls 
which is appreciably less than that of aragonite, 
2'94 : the hardness is about the same, namely, 3^ to 
4 on Mohs's scale. That the arrangement of the 
mineral layers is approximately parallel is evinced 
by the distinctness of the shadow-edges shown on 
examination with the refractometer. Pearls require 
very careful handling, both because they are com- 
paratively soft and therefore apt to be scratched, 
and because they are chemically affected by acids, 
and even by the perspiration from the skin. Acids 
attack only the calcium carbonate, not the organic 
matter ; the well-known story therefore of Cleopatra 
dissolving a valuable pearl in vinegar, which is 
moreover, too weak an acid to effect the solution 
quickly, must not be accepted too literally. Pearls 
are not cut like stones, and therefore as soon as the 
precious bloom has once gone, nothing can be done 
to revive it. Attempts are sometimes made in the 
case of valuable pearls to remove the dull skin and 
lay bare another iridescent layer underneath, but 
the operation is exceedingly delicate. Even with 
the best of care pearls must in process of time 
perish owing to the decay of the organic constituent. 
Pearls that have been discovered in ancient tombs 
crumbled to dust at a touch, and those formerly in 



294 GEM-STONES 

ancient rings have vanished or only remain as a 
brown powder, while the garnets or other stones set 
with them are little the worse for the centuries that 
have passed by. 

The largest known pearl was at one time in the 
famous collection belonging to the banker, Henry 
Philip Hope. Cylindrical in form, with a slight 
swelling at one end, it measures 50 mm. (2 inches) 
in length, and 115 mm. (4! inches) in circumference 
about the thicker, and 83 mm. (3^ inches) about the 
thinner end, and weighs 454 carats. About three- 
quarters of it is white in colour with a fine ' orient,' 
and the remainder is bronze in tint. It is valued at 
upwards of 12,000. A large pearl, 300 carats in 
weight, is in the imperial crown of the Emperor of 
Austria, and another, pear-shaped, is in the posses- 
sion of the Shah of Persia. A beautiful white India 
pearl, a perfect sphere in shape, and 28 carats in 
weight, is in the Museum of Zosima in Moscow ; it 
is known as ' La Pellegrina.' The ' Great Southern 
Cross,' which consists of nine large pearls naturally 
joined together in the shape of a cross, was dis- 
covered in an oyster fished up in 1886 off the beds 
of Western Australia. The collection of jewels in 
the famous Green Vaults at Dresden contains a 
number of pearls of curious shapes. 

Large pearls are sold separately, while the small 
pearls known as ' seed ' pearls come into the market 
bored and strung on silk in ' bunches.' The unit of 
weight is the pearl grain, which is a quarter of a 
carat, and the rate of price depends on the square 
of the weight in grains. The rate per unit or base 
varies from 6d. to 503. according to the shape and 
quality of the pearl. Spherical pearls command 



PL A TE XXX 




PEARL, CORAL, AMBER 295 

the best prices, next the pearl-drops, and lastly the 
buttons ; but whatever the shape, it is imperative 
that the pearl have ' orient,' without which it is 
valueless. The cheaper grades of pearls are sold 
by the carat. 

For use in necklaces and pendants pearls are 
bored with a steel drill, and threaded with silk, 
an easy operation on account of their softness. 
They harmonize well with diamonds. Small pearls 
are often set as a frame to large coloured stones, to 
which they form an admirable foil. Pearls set in 
rings or anywhere where the upper half alone would 
show are generally sawn in halves ; ' button ' pearls 
find an extensive use in modern rings. 

Any mollusc, whether of the bi-valve or the uni- 
valve type, which possesses a nacreous shell, has the 
power of producing pearls, but only two, the pearl- 
oyster, Meleagrina margaritifera, and the pearl- 
mussel, Unio margarifer, repay the cost of systematic 
fishing. The outside of the shell is formed of the 
horny matter called conchiolin ; while the inside is 
composed of two coats, of which the outer consists 
of alternate layers of conchiolin and calcium 
carbonate in its crystallized form, calcite, and the 
inner of the same organic matter, but with calcium 
carbonate in its other crystallized form, aragonite. 
The latter coat forms the nacreous lining known as 
mother- o'-pearl, which is identical in consistency 
with pearl, but somewhat more transparent. The 
iridescence of mother-o'-pearl is due not only to the 
fact that it is composed of a succession of thin 
translucent layers, but also to the fact that these 
layers overlap like slates on a house, and form a 
series of fine parallel lines on the surface; diffrac- 



296 GEM-STONES 

tion therefore as well as interference of light takes 
place, and a similar diffraction phenomenon is dis- 
played even by a cast of the inside of the shell. 
The animal has the property of secreting calcium 
carbonate, which it absorbs from the sea-water, in 
both its crystallized conditions as well as conchiolin. 
At the outer rim it secretes conchiolin, further in 
calcite, and at the very inside aragonite. The shape 
and appearance of a pearl therefore depend on the 
position in which the intruding substance is situated 
within the shell. The most perfect pearl has been 
in intermittent motion in the interior of the mollusc, 
and has received successive coats according to the 
position in which it happened to be. A parasite 
that bores into the shell is walled up at the point of 
entrance, and a wart- or blister-pearl results. The 
thinner the successive coats the finer the lustre. 
Pearls have even been discovered embedded in the 
animal itself. The number of pearls found in 
a shell depends on the number of times the living 
host was compelled to seal up some irritant object, 
and may vary from one up to the eighty-seven which 
are said to have been found in an Indian oyster. 
That an oyster thus distinguished has not led a 
happy existence is testified by the distorted shape 
of its shell, a clue that guides the pearl-fishers in 
their search. Moreover, pearl-oysters never have 
thick nacreous shells, and on the other hand molluscs 
with fine mother-o'-pearl seldom contain pearls. 

Beautiful white and silvery pearls are found in a 
small oyster that lives at a depth of 6 to 13 
fathoms (i 124 m.) in the Gulf of Manaar, off the 
coast of Ceylon. About seven-eighths, however, of 
the pearls that come into the market are obtained 



PLATE XXXI 




PLATE XXXll 





SECTIONS OF CULTURE PEARL 

FIG. I. IN THE OYSTER. FIG. 2. WHEN FINISHED 

A. PEARLY DEPOSIT. B. PIECE OF MOTHER-o'-PEARL INSERTED 
IN THE OYSTER. C. OUTER SHELL OF THE OYSTER. D. MOTHER- 



PEARL, CORAL, AMBER 297 

from a larger oyster which has its home on the 
Arabian coast of the Persian Gulf. These famous 
fisheries have been known since very early times. 
The pearls found here are more yellowish than those 
from Ceylon, but are nevertheless of excellent 
quality. The pearl fisheries off the north-west coast 
of Western Australia and off Venezuela are also not 
unimportant, and fine black pearls have been 
supplied by molluscs from the Gulf of Mexico. 

The Chinese have long made a practice of 
introducing into the shell of a pearl-oyster little 
tin images of Buddha in order that they may be 
coated with the nacreous secretion. The Japanese 
have during recent years made quite an industry of 
stimulating the efforts of the mollusc by cementing 
small pieces of mother-o'-pearl to the interior surface 
of the shell (Plate XXXII, Fig. i); these 'culture' 
pearls, as they are termed, are recognizable by 
examination of the back. About a year has to elapse 
before a coating of a tenth of a millimetre is formed, 
and another two years must pass before the thick- 
ness is doubled. After removal the piece of 
mother-o'-pearl, which is now coated with several 
nacreous layers, is cemented to a piece of ordinary 
mother-o'-pearl, and the lower portion is ground to 
the usual symmetrical shape (Plate XXXII, Fig. 2). 
Blister pearls are often similarly treated. In both 
cases, however, the ' orient ' is deficient in quality. 

The finest mother-o'-pearl is supplied by a 
mollusc found in the sea near the islands lying 
between Borneo and the Philippines, and fine 
material is found at Shark Bay and off Thursday 
Island. 



298 GEM-STONES 



CORAL 

Coral ranks far below pearl and meets with but 
limited appreciation. It is common enough in warm 
seas, but the only kind which finds its way into 
jewellery is the rose or red-coloured coral the 
noble coral, Corallium nobile or rubrum. It consists 
of the axial skeleton of the coral polyp, and is built 
up of hollow tubes fitting one within the other. The 
composition is mainly calcium carbonate with a 
little magnesium carbonate and a small amount of 
organic matter. The former of the mineral sub- 
stances is in the form of calcite, and the crystals 
are arranged in fibrous form radiating at right angles 
to the axis of the coral. The specific gravity varies 
from 2 - 6 to 2'7, being slightly under that of calcite, 
and the hardness is somewhat greater, being about 
3! on Mohs's scale. 

The best red coral is found in the Mediterranean 
Sea off Algiers and Tunis in Africa, and Sicily and 
the Calabrian Coast of Italy. The industry of 
shaping and fashioning the coral is carried on 
almost entirely in Italy. Coral is. usually cut into 
beads, either round or egg-shaped, and used for 
necklaces, rosaries, and bracelets. The best quality 
fetches from 2os. to 303. per carat. 



AMBER 

This fossil resin, yellow and brownish-yellow in 
tint, finds an extensive use as the material for 
mouthpieces of pipes, cigar and cigarette-holders, 
umbrella-handles, and so on, and is even locally cut 
for jewellery, although its extreme softness, its hard- 



PEARL, CORAL, AMBER 299 

ness being only 2| on Mohs's scale, quite unfits it for 
such a purpose. It is only slightly denser than 
water, the specific gravity being about i'io. Since 
the structure is amorphous the refraction is single, 
the index being about i'54O. Amber, being a very 
bad conductor of heat, is perceptibly warm to the 
touch. Its property of becoming electrified by 
friction attracted early attention, and from the 
Greek name for it, rj\eKrpov, is derived our word 
electricity. 

Amber is washed up by the sea off the coasts of 
Sicily and Prussia, and of Norfolk and Suffolk in 
England. The finest examples, which are picked 
up off the shore of Catania in Sicily, are distin- 
guished by a fine bluish fluorescence, resembling 
that seen in lubricating oil ; such pieces command 
good prices. 

A recent resin, pale yellow in colour, known as 
kauri-gum, is found in New Zealand, where it is 
highly valued. 



TABLES 



TABLE I 
Chemical Composition of Gem- Stones 

(a) ELEMENTS 

Diamond C 

(i>) OXIDES 

Corundum A1 2 O S 

Quartz SiO 2 

Chalcedony SiO 8 

Opal . SiO 2 .nII 2 O 

(c) ALUMINATES 

Spinel MgAl 2 O 4 

Chrysoberyl BeAl 2 O 4 

(rf) SILICATES 

Phenakite Be 2 SiO 4 

Dioptase H 2 CuSiO 4 

Peridot Mg 2 SiO 4 

Zircon ZrSiO 4 

Enstatite MgSiO 3 

Diopside CaMg(SiO 3 ) 2 

- Nephrite CaMg,(SiO,) 4 

Sphene CaTiSiO 5 

Benitoite BaTiSi s O 9 

Andalusite ...... Al(AlO)SiO 4 

Kyanite (AlO) 2 SiO 3 

Topaz [Al(F,OH)],Si0 4 

Epidote . . . Ca,(Al,Fe) 2 (AlOH)(SiO 4 ) 3 

Euclase Be(AlOH)SiO 4 

Prehnite H 2 Ca 2 Al,(SiO 4 ) 3 

lolite H 2 (Mg,Fe) 4 Al 8 Si 10 S7 



TABLES 301 

SILICATES continued 

Hessonite ...... Ca 3 Al 2 (SiO 4 ) 3 



Pyrope ...... Mg s Al 2 (Si0 4 ) 3 

Almandine ...... Fe 3 Al 2 (SiO 4 ) 3 

Andradite ...... Ca^jFe^SiOJ,, 

Beryl ....... Be 3 Al 2 (SiO 3 ) 6 

Spodumene ...... LiAl(SiO 3 )j 

Jadeite ...... NaAl(SiO 8 ) 2 

Moonstone ...... KAlSi 3 O 8 

Tourmaline / i*iO s . 3 B/>,.( 9 -*)[(Al,Fe)A].3*[(F* 
M 



Axinite ..... HCa 3 Al 2 B(SiO 4 ) 4 
f(Ca,Mn,Mg,Fe) 2 
\[(Al,Fe)(OH,F)]Si a 7 

(e) PHOSPHATES 

Beryllonite ...... NaBePO 4 

Apatite ..... Ca 6 (F,Cl)(P0 4 ) 3 

Turquoise . . . CuOH.6[Al(OH) 2 ].H 6 .(PO 4 ) 4 



TABLE II 
Colour of Gem-Stones 

Colourless and White. Diamond, corundum (white 
sapphire), topaz, quartz (rock-crystal), zircon (when 
'fired'), moonstone; rarely beryl, tourmaline; 
among the less common species, phenakite, 
spodumene (colourless kunzite), beryllonite. 

Yellow. Diamond, topaz, corundum (yellow sapphire), 
quartz (citrine, Scotch or occidental topaz), tourma- 
line, zircon, sphene, spodumene, beryl. 

Pink and Lilac. Corundum (pink sapphire), spinel 
(balas-ruby), tourmaline (rubellite), topaz (usually 
when 'fired'), spodumene (kunzite), beryl (mor- 
ganite), quartz (rose-quartz). 

Red. Corundum (ruby), garnet (pyrope, almandine), 
spinel (balas-ruby), tourmaline (rubellite), zircon, 
opal (fire-opal). 



302 



GEM-STONES 



Green. Beryl (emerald, aquamarine), peridot, cor- 
undum, tourmaline, chrysoberyl (including alex- 
andrite), zircon, garnet (demantoid) ; among less 
common species, spodumene (hiddenite), euclase, 
diopside, idocrase, epidote, apatite, obsidian ; 
rarely diamond ; also semi-opaque, turquoise, jade. 

Blue. Corundum (sapphire), spinel, topaz, tourmaline, 
zircon; among the less common species, kyanite, 
iolite, benitoite, apatite; rarely diamond; also 
semi-opaque, turquoise, lapis lazuli, sodalite. 

Violet and Purple. Quartz (amethyst), corundum 
(oriental amethyst), spinel (almandine- spinel), 
garnet (almandine), spodumene (kunzite), apatite. 

Brown. Diamond, tourmaline, quartz (smoky-quartz); 
among the less common species, andalusite, 
axinite, sphene. 



TABLE III 

Refractive Indices of Gem-Stones* 



^pai . 

Moonstone 






'S3 




* 454 




'54 


Iolite 






'543 








'SSI 


Quartz . 






544 








'553 


Beryllonite 






'553 








565 


Beryl . 






578 








585 


Turquoise 






1-61 








I-6 5 


Topaz . 






1-618 








I'627 


Andalusite 






1-632 








I '643 


Tourmaline 






1-626 








1-651 


Apatite . 






1-642 








1-646 


Phenakite 






1-652 








1-667 


Euclase . 






1-651 








1-670 


Spodumene 






1-660 








1-675 


Enstatite . 






1-665 








1-674 



1 The least and the greatest of the refractive indices of 
doubly refractive species are given. 



TABLES 



303 



Peridot . 

Axinite . . . 

Diopside . 

Idocrase . 

Spinel . 

Kyanite . 

Epidote . 

Garnet (Hessonite) . 

Chrysoberyl . 

Garnet (Pyrope) 

Benitoite . 

Corundum 

Garnet (Almandine) 

Zircon (a) 

Garnet (Demantoid) 

Sphene . 

Zircon (b) 

Diamond 



1-674 
1-685 
1714 



172 

1735 

1746 

1757 
1-761 



1-901 
1-927 



1726 

1745 
1755 



1790 
1-815 

i-88 



2-417 



1-697 
1-684 
1705 
1719 

173 
1766 

1753 

1-804 
1770 



I-985 
1-980 



Moonstone 

Quartz 

Beryl 

Topaz 

Chrysoberyl 

Tourmaline 

Spodumene 

Corundum 

Peridot 



TABLE IV 
Colour-Dispersion of Gem-Stones 1 

. -012 Spinel .... 'O2O 

. '013 Garnet (Almandine) . . '024 

. -014 Garnet (Pyrope) . . '027 

. '014 Garnet (Hessonite) . . -028 

. "015 Zircon .... '038 

. '017 Diamond .... '044 

. -017 Sphene .... -051 

. -018 Garnet (Demantoid) . . '057 
. 'O2O 



TABLE V 

Character of the Refraction of Gem-Stones 
(a) SINGLE 

Diamond, spinel, garnet, opal. 

Diamond and garnet frequently display local double refraction. 

1 The dispersion is the difference of the refractive indices correspond- 
ing to the B and G lines of the solar spectrum. The value for crown- 
glass is 'Oi6. 



304 



GEM-STONES 



Quartz 
Phenakite. 



Apatite 
Idocrase 
Beryl 



Chrysoberyl 
Topaz 
Enstatite . 
Spodumene 



Moonstone 
lolite 
Axinite 
Andalusite 



(6) UNIAXIAL, POSITIVE 
. -009 I Benitoite . 
. '015 I Zircon (b). 
Quartz exhibits circular polarization. 

(c) UNIAXIAL, NEGATIVE 



004 
005 
007 



Corundum 
Tourmaline 



(</) BIAXIAL, POSITIVE 

. . '007 Euclase . 

. '009 Diopside . 

. '009 Peridot 

. . "015 Sphene . 

(e) BIAXIAL, NEGATIVE 



006 
008 
oio 
on 



Beryllonite 
Kyanite . 
Epidote . 



047 
053 



009 
025 



019 
020 
038 

084 



01? 
016 
031 



TABLE VI 

Dichroism of Gem-Stones 
(a) STRONG 

Corundum, tourmaline, alexandrite, spodumene, and- 
alusite, iolite, epidote, axinite. 

() DISTINCT 

Emerald, topaz, quartz, peridot, chrysoberyl, enstatite, 
euclase, idocrase, kyanite, sphene, apatite. 



Beryl, diopside. 



WEAK 



TABLES 



305 



TABLE VII 

Specific Gravities of Gem-Stones 



Opal 


. 2-15 


Peridot . 


. 3-40 


Moonstone 


. 2-57 


Idocrase . 


3 '4 


lolite 


. 2-63 Sphene . 


3 '4 


Quartz 


. 2-66 Diamond . 


VS2 


Beryl 




T 


3 j* 


Turquoise 


. 2-82 


Spinel 


3'53 
. 360 


Beryllonite 


. . 2-84 


Kyanite . 


3'6i 


Phenakite . 


2-99 


Garnet (Hessonite) . 


. 3'6i 


Euclase . 


3 - o? 


Benitoite . 


3 '64 


Tourmaline 


. 3'io 


Chrysoberyl 


373 


Enstatite . 


3'i 


Garnet (Pyrope) 


378 


Andalusite 


3-18 


Garnet (Demantoid) . 


3^4 


Spodumene 


3'i8 


Corundum 


. 4-03 


Apatite . 


. 3'20 


Garnet (Almandine) . 


4 '05 


Axinite 


. . 3-28 


Zircon (a) . 


. 4-20 


Diopside . 


. 3-29 


Zircon (b) . 


4-69 


Epidote . 


3'37j? 







TABLE VIII 

Degrees of Hardness of Gem-Stones 

5. Kyanite (5-7), apatite, lapis lazuli 
5^. Enstatite, beryllonite, sphene 

6. Opal, moonstone, turquoise, diopside 

G. Spodumene, peridot, garnet (demantoid), benitoite, 
idocrase, epidote, axinite, jade (nephrite) 

7. lolite, quajtz, tourmaline, jade (jadeite) 
7^. Garnet (hessonite, pyrope) 

7^. Beryl, garnet (almandine), zircon, phenakite, euclase, 
andalusite 

8. Topaz, spinel 
8. Chrysoberyl 

9. Corundum 
10. Diamond 



306 



GEM-STONES 



TABLE IX. DATA 
Densities of Water and Toluol at Ordinary Temperatures 



TEMPERATURE 


WATER 


TOLUOL 


Centigrade 
I? 


Fahrenheit 
57'2 


0-9994 


0-8697 


15 


59 '0 


0-9992 


0-8687 


16 


60-8 


0-9990 


0-8677 


17 


62-6 


0-9988 


0-8667 


18 


64-4 


0-9986 


0-8657 


19 


66-2 


0-9985 


0-8647 


20 


68-0 


0-9983 


0-8637 


21 


69-0 


0-9981 


0-8627 


22 


7 r6 


0-9979 


0-8617 


23 


73'4 


0-9977 


0-8607 



English carat 
Metric carat 
oz Av 


= 0-2053 gram 
= 0-2000 (one-fifth) gram 
28*35 grams 


Ib. Av. 
inch 
foot . 
yard . 
mile . 


= 0*4536 kilogram 
= 25*4 millimetres 
= 0*3048 metre 
= 0*9144 metre 
= I -6093 kilometre 



INDEX 



Absorption, 53, 59 


Baroque, Barrok, pearls, 292 


Absorption spectra, 59 


Bastite, 272 


Achroite, 220, 221 


Benitoite, 267 


Adularia, 255 


Berquem, Louis de, 90, 161 


Agate, 247 


Beryl, 184 


Akbar Shah diamond, 163 


Beryl lonite, 270 


Alalite, 272 


Bezel facet, 92 


Albite, 254 


Biaxial double refraction, 45, 49, 


Alexandrite, 54, 60, 233 


57 


Scientific, 122 


Bisectrix, 45, 49 


Almandine, 60, 214 


Black diamond, 129 


Oriental, 112, 172 


Black lead, 129 


spinel, 112, 204 


Black opal, 249, 250 


Amazon-stone, 255 


Black Prince's ruby, 206 


Amber, 83, 298 


Blister-pearl, 296 


Amethyst, 239, 242 


Bloodstone, 247 


Oriental, III, 172, 239 


Blue felspar, 255 


Anatase, 281 


Blue ground, 143, 147 


Andalusite, 274 


Blue John, 285 


Andradite, 216 


Boart, 103, 129, 133 


Anomalous refraction, 47 


Bohemian garnet (pyrope), 207, 


Anorthite, 254 


212 


Apatite, 279 
Apophyllite, 290 


Bone turquoise, 259 
Boodt, A. B. de, 132, 213 


Aquamarine, 184, '93 


Borgis, Hortensio, 161 


Arizona-ruby, 213 


Borneo stones, 154, 170 


Artificial stones, 124 


Bort, v. Boart, 103, 129, 133 


Asteria, 38, 177 


Bottle-stone, 284 


Asterism, 38 


Boule, 1 18 


Australia stones, 154, 174,182,195, 


Bowenite, 263 


213,216,227,232, 252, 288 


Braganza diamond, 170 


Austrian Yellow diamond, 165 


Brazil stones, 138, 165, 166, 169, 


Aventurine, 240, 241 


ig^etseq., 201, 215, 223, 


Axes, Crystallographic, 9 


236, 243, 244, 248, 266, 


Optic, 49 


269, 270, 274 


Axinite, 278 


Brazilian emerald, HI, 22O, 221 


Azure-quartz, 244 


peridot, 221 


Azurite, 287 


sapphire, ill, 221 




topaz, III, 197 


Balas-ruby, 203 
Barnato, Barnett, 145 


Brilliant form of cutting, 92 
Brilliant, Scientific, 122 



308 



GEM-STONES 



Bristol diamonds, 243 


Colour dispersion, 20, 97 


Bruting, 100 


Conchiolin, 293 


Burma stones. 178. 205, 223, 227, 


Coral, 298 


263 


Cordierite, 266 


Button-pearl, 295 


Cornish diamonds, 243 


Byes, By waters, 136, 150 


Corundum, 172 




Crocidolite, 39, 240 


Cabochon form of cutting, 88 


Crookes, Sir William, 132, 153 


Cacholong, 251 


Cross facet, 93 


Cairngorm, 239 


Crystal, 678 


Callaica, callaina, callais, 258 


Rock-, 97 


Calcite, 40, 289 


Cubic system, 8 


California stones, 156, 195, 202, 


Culet facet, 93 


224, 259, 265, 267, 275 


Cullinan diamond, 94, 100, 168 


Californite, 264, 275 


Culture pearls, 297 


Cape-ruby, 213 


Cumberland diamond, 164 


Carat weight, 72, 84 


Cyanite (Kyanite), 79, 273 


Carbon, 129 


Cymophane, 234 


Carbonado, 129 




Carborundum, 105 


Darya-i-nor diamond, 162 


Carbuncle, 89, 215 


De Beers diamonds, 167 


Carnelian, 247 


Demantoid, 216 


Cascalho, 139 


Density, 63 


Cassiterite, 281 


Deviation, Minimum, 30 


Cat's-eye (chrysoberyl), 38, 90, 


Diamond, Characters of, 128 


233 


cutting, 90 


(quartz), 39, 90, 240 


gauges, 86 


(tourmaline), 39, 219 


Glaziers', 135 


Hungarian, 244 
Ceylon stones, 181, 195, 201, 205, 


mining, 146 
Occurrence of, in 


212, 215, 216, 223, 232, 


Borneo, 154 


236, 237, 243, 244, 255, 


Brazil, 139 


267, 274, 279, 284 


German South-West Africa, 


Ceylonese peridot (tourmaline), 


ISS 


221 


India, 138 


Ceylonite, 204 


New South Wales, 154 


Chalcedony, 246 


Rhodesia, 155 


Chatoyancy, 38 


South Africa, 139 


Chert, 247 


Origin of, 151 


Chessylite, 287 


-point, 91 


Chrysoberyl, 233 


-rose, 92 


Chrysocolla, 288 


-table, 91 


Chrysolite (chrysoberyl), 233 


Diamonds, Classification of, 136, 


(peridot), 225 


149 


Chrysoprase, 247 


Historical, 157 


Church, Sir Arthur, 6l, 211, 231 


Prices of, 135 


Cinnamon-stone, 211 


Dichroism, >>S 


Citrine, 239 


Dichroite, 266 


Cleavage, 80, 100, 149 
Close goods, 149 


Dichroscope, 55 
Diffusion column, 65 


Colenso diamond, 131 


Diopside, 272 


Colour, 53 


Dioptase, 280 



INDEX 



309 



Dispersion, Colour, 20, 24, 97 


Graphite, 129 


Disthene, 273 


Greaser, 149 


Dop, 102 


Great Mogul diamond, 161 


Double refraction, 28, 40 


Great Southern Cross group of 


Doublet, 125 


pearls, 294 


Dresden diamond, 171 


Great Table diamond, 162 


Drop-stone, 94 


Great White diamond, 167 


Duke of Devonshire's emerald, 191 


Green garnet, 271 




Greenstone, 261 


Edwardes ruby, 175 


Grossular, 21 1 


Electrical characters, 82 




Emerald, 89, 184 


Habit, 12 


Brazilian, 220, 221 


Hardness, 78 


Evening, 225 


Haiiynite, 286 


Oriental, III, 172 


Heavy liquids, 64 


Scientific, 122 


Hematite, 282 


Uralian, 216 


Hessonite, 211 


Emeraldine, 247 


Hexagonal system, 10 


Emery, 175 


Hiddenite, 266 


English Dresden diamond, 166 


Hope cat's-eye, 237 


Enstatite, 271 


chrysolite, 237 


Epidote, 275 


diamond, 170 


Essence d'Orient, 126 


pearl, 294 


Essonite (Hessonite), 21 1 


sapphire, 1 21 


Euclase, 269 


Hornstone, 247 


Eugenie diamond, 164 


Hungarian cat's-eye, 244 


Evening emerald, 225 
Excelsior diamond, 1167 


Hyacinth, 211, 228 
Hydrophane, 250 


Extinction, 45 


Hydrostatic weighing, 72 




Hypersthene, 271 


Faceting machine, 105 




False topaz, 239 


Iceland-spar, 40, 44 


Felspar, 254 


Idocrase, 274 


Fire, 20, 96 


Imitation stones, 124 


Fire-marble, 289 


Imperial diamond, 167 


Fire-opal, 251 


Index of refraction, 16 


Flats, 150 


India stones, 137, 181, 194, 215, 


Fleches d'amour, 240 


243, 244, 248, 290 


Flint, 247 


Indicators, 65 


Floors, 147 


Indicolite, 221 


Fluor, 285 


Interference of light, 39, 48 


Fremy, E., 115 


lolite, 266 




Iris, 240 


Garnet, 207 


Isle of Wight diamonds, 243 


Green, 271 


Isomorphous replacement, 13, 19 


Gaudin, M. A. A., 115 




Gauges, Diamond, 86 
Girdle, 92 


acinth, 2 1 1, 228 
ade, 260 


Glass, 7, 124 


] adeite, 262 


Gnaga Boh ruby, 180 
Goniometer, 30 


_ argoon, 228 
] asper, 247 


Grain, Pearl, 86 


| ehan Ghir Shah diamond, 163 



3 io 



GEM-STONES 



Jigger, 149 
Jubilee diamond, 167 


Nacre, 292 
Napoleon diamond, 164 
Nassak diamond, 163 




Kauri-gum, 299 


Negative double refraction, 45 




Khiraj-i-Alam ruby, 206 


Nephrite, 261 




Kimberlite, 152 


Nicol's prism, 44 




King topaz, 181, 201 
Klein's solution, 67 


Nizam diamond, 162 




Koh-i-nor diamond, 137, 158 


Obsidian, 283 




Kunz, Dr. G. F., 186, 224, 262, 


Occidental topaz, III, 239 




265 


Odontolite, 259 




Kunzite, 265 


Off-coloured diamonds, 130 




Kyanite, 79, 273 


Olivine (demantoid), 216 






(peridot), 225 




Labradorite, 255 


Onyx, 247 




La Pellegrina pearl, 294 


Opal, 39, 249 




Lapis lazuli, 286 


Fire, 251 




Lazurite, 286 


-matrix, 251 




Lozenge facet, 93 


Opalescence, 39 




Lumachelle, 289 


Optical anomalies, 47 




Lustre, 37 


Optic axes, 49 
Oriental almandine, 112, 172 




Maacles, Macles, 12, 150 


amethyst, III, 172 




Madagascar stones, 195, 224, 243, 


emerald, in, 172 




265, 266 


topaz, III, 172 




Malachite, 287 
Malacolite, 272 


Orient of pearls, 292 
Orloff diamond, 160 




Manufactured stones, 113 


Ortboclase, 254 




Marble, 289 


Orthorhombic system, 1 1 




Mattan diamond, 155, 170 






Matura diamonds, 232 


Pacha of Egypt diamond, 1 65 




Mazarin, Cardinal, 92 


Paste, 47, 124 




Meerschaum, 288 


Paul I diamond, 171 




Melee, 136 


Pavilion, 93 




Methylene iodide, 26, 66 


Pavilion facet, 93 




Metric carat, 85, 87 


Pear-drop pearls, 292 




Milky-quartz, 240 
Minimum deviation, 30 


Pear-eye pearls, 292 
Pearl, 291 




Mocha-stone, 247 


grain, 86 




Moe's gauge, 87 
Mohs's scale of hardness, 78 


imitations, 126 
Pendeloque, 94 




Moissan, Henri, 153 


Peridot, 225 




Moldavite, 283 


Brazilian, 221 




Monoclinic system, 1 1 


Ceylonese, 221 




Moon of the Mountains diamond, 


Peruzzi, Vincenzio, 92 




162 


Phenakite, 269 




Moonstone, 39, 255 


Pigott diamond, 164 




Morganite, 186, 195 


Pipes, 152 




Moroxite, 279 
Moss-agate, 247 


Pistacite, 275 
Pitt diamond, 100, 159 




Mother-of-emerald, 240 


Plasma, 247, 264 




Mother-o'-pearl, 292 Pleochroism, 57 





INDEX 



Pleonaste, 204 


Schorl, 221 


Pliny, 6, 88, 138, 184, 191, 241, 


Scientific alexandrite, 122 


249 


brilliant, 122 


Polar Star diamond, 163 


emerald, 122 


Polarization, 42 


topaz, 121 


Porter-Rhodes diamond, 166 


Scotch topaz, 239 


Positive double refraction, 45 


Seed pearls, 294 


Prase, 240, 247 


Serpentine, 289 


Prehnite, 278 


Setting of gem-stones, 107 


Pycnometer, 75 


Shah diamond, 163 


Pyrites, 282 


Sheen, 39 


Pyrope, 212 


Shepherd's Stone diamond, 163 




Siam stones, 180 


Quartz, 50, 238 


Siberia and Asiatic Russia stones, 


Quoin facet, 93 


182, 188, 194, 201, 217, 




223, 236, 244, 256, 262, 


Rainbow-quartz, 240 


269, 270, 287 


Reconstructed stones, 116 


Siberite, 221 


Reef, 144 


Siderite, 244 


Reflection of light, 14 


Silver-thallium nitrate, 69 


Refraction of light, 15 


Skew facet, 93 


Refractive index, 16 


Skill facet, 93 


Refractometer, 22, 50 


Smoky quartz, 240 


Regent diamond, 100, 159 


Snell's laws, 16 


Retgers's salt, 69 


Soapstone, 288 


Rhodes, Cecil]., 145 


Sodalite, 286, 287 


Rhodesia stones, 155, 183, 213, 236 


Sonstadt's solution, 67 


Rhodolite, 62, 214 


South Africa stones, 139 et seq., 


Rhodonite, 287 


166, 167 et seq., 213, 232, 


Rock-crystal, 97, 239 
Rock-drill, 134 


244, 264, 271 
Spanish topaz, 239 


Rontgen rays, 83 
Rose form of cutting, 91 


Specific gravity, 63 
Specific-gravity bottle, 75 


Rose-quartz, 240 
Rospoli sapphire, 182 
Rotation of plane of polarization, 


Spectroscope, 59 
Spectrum, 20, 25 
Spectium, Absorption, 59 


50 


Spessartite, 216 


Rubellite, 220, 223 


Sphene, 276 


Rubicelle, 203 


Spinel, 203 


Ruby, 98, 110, 172 


Spodumene, 265 


Balas-, 203 


Spotted stones, 149 


Cape-, 213 


Star-facet, 92 
Star of Africa diamond, 168 


Sancy diamond, 161 


Star of Este diamond, 165 


Sapphire, 98, no, 172 


Star of Minas diamond, 169 


Brazilian (tourmaline), 221 


Star of South Africa diamond, 


-quartz, 244 


141, 166 


Water- (iolite), 266 


Star of the South diamond, 139, 


Water- (topaz), 201 


165 


Sard, 247 


Starstones, 38, 177 


Sardonyx, 247 


Steatite, 288 


Saussurite, 263 


Step form of cutting, 98 



312 



GEM-STONES 



Stewart diamond, 166 Turquoise-matrix, 2& 


btrass, 124 
Sunstone, 255 
Synthetical stones, 113 


Tuscany diamond, 165 
Twinning, 12, 47 


Syriam, Syrian, garnet, 215 


Uniaxial double refraction, 45, 48 


Table facet, 92 
Table form of cutting, 91 
Tavernier, J. B., 91, 129, 137, 161, 


Uralian emerald, 217 
Uvarovite, 218 


162, 170 




Templet facet, 92 


Variscite, 259 


Tetragonal system, 9 


Verdite, 264 


Thulite, 289 


Verneuil, A. V. L., 116 


Tiffany diamond, 171 


Vesuvianite, 274 


Tiger's-eye, 39, 240 


Victoria diamond, 167 


Timur-ruby, 206 


Violane, 287 


Titanite, 276 




Topaz, 197 
Brazilian, 197 
False, 239 
Occidental, ill, 239 
Oriental, in, 173 
Scientific, 121 
Scotch, 239 
Spanish, 239 
Topazolite, 216 
Total-reflection, 18, 21 


Wart-pearl, 296 
Water (of diamonds), 129 
(of pearls), 292 
Water-chrysolite, 284 
-sapphire (iolite), 266 
-sapphire (topaz), 201 
White opal, 249 
White Saxon diamond, 165 
Wollaston.W. H., 133 


Tourmaline, 43, 219 




Trap form of cutting, 98 


X-rays, 83 


Trichroism, 57 




Triclinic system, 12 
Triplet, 126 


Yellow ground, 143 


Turquoise, 257 


Zircon, 228 



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COIL OF CARNB, THE. John Oxenham. 
CONVERT, THE. Elizabeth Robins. 
COUNSEL OF PERFECTION, A. Lucas Malet. 
CROOKED WAY, THE. William Le Queux. 
DAN RUSSEL THE Fox. E. CE. Somerville 

and Martin Ross. 

DARNELBY PLACE. Richard Bagot. 
DEAD MEN TELL no TALES. E. W. Hor- 

nung. 

DEMETER'S DAUGHTER. Eden Phillpotts. 
DEMON, THE. C. N. and A. M. Williamson. 



DESERT TRAIL, THE. Dane Coolidge. 
DEVIL DOCTOR, THE. Sax Rohmer. 
DOUBLE LIFE OF MR. ALFRED BURTON, 

THE. E. Phillips Oppenheim. 
DUKE'S MOTTO, THE. J. H. McCarthy. 
EMMANUEL BURDEN. Hilaire Belloc. 
END OF HER HONEYMOON, THE. Mrs. 

Belloc Lowndes. 

FAMILY, THE. Elinor Mordaunt. 
FIRE IK STUBBLE. Baroness Orczy. 
FIREMEN HOT. C. J. CDTCLIFFE HYNE. 
FLOWER OF THE DUSK. Myrtle Reed. 
GATE OF THE DESERT, THE. John Oxenham. 
GATES OF WRATH, THE. Arnold Bennett. 
GENTLEMAN ADVENTURER, THE. H. C. 

Bailey. 

GOLDEN CENTIPEDE, THE. Louise Gerard. 
GOLDEN SILENCE, THE. C. N. and A. M. 

Williamson. 

GOSSAMER. George A. Birmingham. 
GOVERNOR OF ENGLAND, THE. Marjorie 

Bowen. 

GREAT LADY, A. Adeline Sergeant. 
GREAT MAN, A. Arnold Bennett. 
GUARDED FLAME, THE. W. B. Maxwell 
GUIDING THREAD, THE. Beatrice Harraden. 
HALO, THE. Baroness von Hntten. 
HAPPY HUNTING GROUND, THE. Alice 

Perrin. 

HAPPY VALLEY, THE. B. M. Croker. 
HEART OF HIS HEART. E. Maria Albanesi. 
HEART OF THE ANCIENT WOOD, THE. 

Charles G. D. Roberts. 
HEATHER MOON, THE. C. N. and A. M. 

Williamson. 

HERITAGE OF PERIL, A. A. W. Marchmont. 
HIGHWAYMAN, The. H. C. Bailey. 
HILLMAN, THE. E. Phillips Oppenheim. 
HILL RISE. W. B. Maxwell. 
HOUSE OF SERKAVALLB, THE. Richard 

Bagot. 

HYENA OF KAI.LU, THE. Louise Gerard. 
ISLAND PRINCESS, His. W. Clark Russell. 



30 METHUEN AND COMPANY LIMITED 

Methuen's Cheap Novel* continued. 

JANE. Marie CorellL 
JOHANNA. B. M. Croker. 
JOSEPH. Frank Danby. 



JOSHUA DAVIDSON, COMMUNIST. 
Linton. 



E. Lynr 



Joss, THE. Richard Marsh. 

KINSMAN, THE. Mrs. Alfred Sidgwick. 

KNIGHT OF SPAIN, A. Marjorie Bowen. 

LADY BETTY ACROSS THB WATER. C. N. 
and A. M. Williamson. 

LALAGE'S LOVERS. George A. Birmingham. 

LANTERN BEARERS, THE. Mrs. Alfred Sidg- 
wick. 

LAURISTONS. John Oxenham, 

LAVENDER AND OLD LACK. Myrtle Reed. 

LIGHT FREIGHTS. W. W. Jacobs. 

LODGER, THE. Mrs. Belloc Lowndes. 

LONG ROAD, THE. John Oxenham. 

LOVE AND LOUISA. E. Maria Albanesi. 

LOVE PIRATE, THE. C. N. and A. M. 

Williamson. 

MARY ALL- ALONE. John Oxenham. 
MASTER OF THE VINEYARD. Myrtle Reed. 
MASTER'S VIOLIN, THE. Myrtle Reed. 
MAX CARRADOS. Ernest Bramah. 
MAYOR OF TROY, THE. "Q." 
Muss DECK, THE. W. F. Shannon. 
MIGHTY ATOM, THB. Marie Corelli. 
MIRAGE. E. Temple Thurston. 
MISSING DELORA, THE. E. Phillips Oppcn- 

heim. 
MR. GREX OF MONTH CARLO. E. Phillips 

Oppenheim. 

MR. WASHINGTON. Marjorie Bowen. 
MRS. MAXON PROTESTS. Anthony Hope. 
MRS. PETER HOWARD. Mary E. Mann. 
MY DANISH SWEETHEART. W. Clark 

Russell. 
MY FRIEND THE CHAUFFTCUR. C. N. and 

A. M. Williamson. 

MY HUSBAND AND I. Leo Tolstoy. 
MY LADY OF SHADOWS. John Oxenham. 
MYSTERY OF DR. FU-MANCHU, THE. Sax 

Rohmer. 
MYSTERY OF THE GREEN HEART, THE. 

Max Pemberton. 
NINE DAYS' WONDEB, A. B. M. Croker. 



NINK TO Srx-THiSTT. W. Pett Ridge. 
OCEAN SLEUTH, THE. Maurice Drake. 
OLD ROSE AND SILVER. Myrtle Reed. 
PATHS OF THE PRUDENT, THE. J. S. Fletcher. 
PATHWAY OF THE PIONEER, THE. Doll 

Wyllarde. 

PEGGY OF THB BARTONS. B. M. Croker. 
PEOPLE'S MAN, A. E. Phillips Oppenheim, 
PETER AND JANE. S. Macnaughtan. 
POMP OF THE LAVILETTES, THE. Sir Gilbert 

Parker. 

QUEST OF GLORY, THB. Marjorie Bowen. 
QUEST OF THB GOLDEN ROSE, THE. John 

Oxenbam. , 

REGENT, THE. Arnold Bennett. 
REMINGTON SENTENCE, THB. W. Pett 

Ridge. 

REST CURE, THB. W. B. MaxwelL 
RETURN OF TARZAN, THE. Edgar Rice 

Burroughs. 

ROUND THE RED LAMP. Sir A. Conan Doyle. 

ROYAL GEORGIB. S. Baring-Gould. 

SAID, THB FISHERMAN. Marmaduke Pick- 

thall. 
SALLY. Dorothea Conyers, 

SALVING OF A DERELICT, THE. Maurice 
Drake. 

SANDY MARRIED. Dorothea Conyers. 

SEA CAPTAIN, THE. H. C. Bailey. 

SEA LADY, THE. H. G. Wells. 

SEARCH PARTY. THE. George A. Birmingham. 

SECRET AGENT, THE. Joseph Conrad. 

SECRET HISTORY. C. N. and A. M. William- 
son. 

SECRET WOMAN, THE. Eden Phillpotts. 

SET IN SILVER. C. N. and A. M. William- 
son. 

SEVASTOPOL, AND OTHER STORIES, Leo 
Tolstoy. 

SEVERINS, THE. Mrs. Alfred Sidgwick. 

SHORT CRUISES. W. W. Jacobs. 

SI-FAN MYSTERIES, THE. Sax Rohmer. 

SPANISH GOLD. George A. Birmingham. 

SPINNER IN THE SUN, A. Myrtle Reed. 

STREET CALLED STRAIGHT, THE. Basil 
King. 

SUPREME CRIME, THE. Dorothea Gerard. 

TALES OF MEAN STREETS. Arthur Morrison. 

TARZAN OF THIS APKS. Edgar Ric Btx- 



FlCTIOW 



Hethuon** Cheap Xov tin continued. 

TERESA OF WATLING STREET. Arnold 

Bennett. 

THERE WAS A CROOKED MAN. Dolf Wyllarde. 
TYRANT, THE. Mrs. Henry de la Pasture. 
UNDER WESTERN EVES. Joseph Conrad. 
UNOFFICIAL HONEYMOON, THE. Dolf 

Wyllarde. 
VALLEY OF THB SHADOW, THB. William 

Le Queux. 

VIRGINIA PERFECT. Peggy Webling. 
WALLET OF KAI LUNG. Ernest Bramah. 
WAR WEDDING, THB. C. N. and A. M. 

Williamson. 

WARE CASE, THE. George PleydelL 
WAY HOMK, THE. Basil King. 



WAT OF THESE WOMEN, THK. E. Phillips 

Oppenheim. 

WEAVER OF DREAMS, A. Myrtle Reed. 
WEAVER OF WEBS, A. John Oxenham. 
WEDDING DAY, THE. C. N. and A. M. 

Williamson. 

WHITE FANG. Jack London. 
WILD OLIVE, THB. Basil King. 
WILLIAM, BY THE GRACE OF GOD. Marjorie 

Bowen. 
WOMAN WITH THB FAN, THE. Robert 

Hichens. 

WO* Maurice Drake. 

WONDER OF LOVE, THE. E. Maria Albanesi. 
YELLOW CLAW, THE. Sax Rohmer. 
YELLOW DIAMOND, THE. Adeline Sergeant. 



Methuen's One and Threepenny Novels 



BARBARA RESELL. Mrs. Eelloc Lowndes. 
Bv STROKE OF SWORD. Andrew Balfour. 
DERRICK VAUGHAN, NOVELIST. Edna 

LyalL 
HOUSE OF WHISPERS, THK. William Le 

Queux. 
INCA'S TREASURE, THE E. Glanville. 



KATHERINK THB ARROGANT. Mrs. B. M. 
Croker. 

MOTHER'S SON, A. B. and C. B. Fry. 
PROFIT AND Loss. John Oxenham. 
RED DERELICT, THB. Bertram Mitford. 
SIGN OF THK SPIDER, THB. Bertram Mitford. 



37/6/19. 



PRINTSP BY MORRISON AND GIBB LIMITED, EDINBURGH 



UC SOUTHERN REGIONAL 



A 000 020 479 2 



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