no other stone, though it is possible, by heating alumina coloured with
oxide of iron and perhaps also a trace of oxide of chromium to a very
high temperature for a considerable time, and then cooling very slowly,
to obtain a ruby which is nearly the same in its structure as the real
gem; its specific gravity and hardness may perhaps be to standard, and
when properly cut, its brilliance would deceive all but an expert. And
as in some real rubies there are found slight hollows corresponding or
analogous to the bubbles found in melted glass, it becomes a matter of
great difficulty to distinguish the real from the imitation by such
tests as hardness, specific gravity, dichroism, and the like, so that in
such a case, short of risking the ruin of the stone, ordinary persons
are unable to apply any convincing tests. Therefore, only the expert can
decide, by his appreciation of the delicate shade of difference in the
light of a true ruby and that of an excellent imitation, and by the
distribution of the colour, which--however experienced the chemist may
be, or with what care the colouring matter may have been incorporated in
the mass--has been found impossible of distribution throughout the body
of an artificial stone so perfectly and in the same manner and direction
as nature herself distributes it in the genuine. This alone, even in the
closest imitations, is clear to the eye of the expert, though not to the
untrained eye, unless the stone is palpably spurious. To one who is
accustomed to the examination of precious stones, however perfect the
imitation, it is but necessary to place it beside or amongst one or more
real ones for the false to be almost instantly identified, and that with
certainty.

_The Sapphire._

The Sapphire is not so easy to imitate, as its hardness exceeds that of
the ruby, and imitations containing its known constituents, or of glass,
are invariably softer than the natural stone. As before remarked, almost
any form of corundum other than red is, broadly, called sapphire, but
giving them their strictly correct designations, we have the olivine
corundum, called "chrysolite" (oriental), which is harder than the
ordinary or "noble" chrysolite, sometimes called the "peridot." The
various yellow varieties of corundum take the name of the "oriental
topaz," which, like most, if not all, the corundum varieties, is harder
than the gem which bears the same name, minus the prefix "oriental."
Then we have the "amethyst" sapphire, which varies from a red to a blue
purple, being richer in colour than the ordinary amethyst, which is a
form of violet-coloured quartz, but the corundum variety, which, like
its companions, is called the "oriental" amethyst, is both rarer and
more precious. A very rare and extremely beautiful green variety is
called the oriental emerald. The oriental jacinth, or hyacinth, is a
brown-red corundum, which is more stable than the ordinary hyacinth,
this latter being a form of zircon; it changes colour on exposure to
light, which colour is not restored by subsequent retention in darkness.

The blue sapphire is of all shades of blue, from cornflower blue to the
very palest tints of this colour, all the gradations from light to dark
purple blues, and, in fact, so many shades of tone and colour that they
become almost as numerous as the stones. These stones are usually found
in similar situations to those which produce the ruby, and often along
with them. The lighter colours are usually called females, or feminine
stones, whilst the darker ones are called masculine stones. Some of
these dark ones are so deep as to be almost black, when they are called
"ink" sapphires, and if inclining to blue, "indigo" sapphires, in
contradistinction to which the palest of the stones are called "water"
sapphires. The colouring matter is not always even, but is often spread
over the substance of the stone in scabs or "splotches," which rather
favours imitation, and, where this unevenness occurs, it may be
necessary to cut or divide the stone, or so to arrange the form of it
that the finished stone shall be equally blue throughout.

In some cases, however, the sapphire may owe its beauty to the presence
of two, three or more colours in separate strata appearing in one stone;
such as a portion being a green-blue, another a cornflower blue, another
perfectly colourless, another a pale sky blue, another yellow, each
perfectly distinct, the stone being cut so as to show each colour in its
full perfection.

This stone, the sapphire, is hardness No. 9 (see "Hardness" table), and
therefore ranks next to the diamond, which makes it a matter of great
difficulty to obtain an imitation which is of the same specific gravity
and of the same degree of hardness, though this has been done. Such
stones are purchasable, but though sold as imitations at comparatively
low price, and the buyer may consider them just as good as the real gem,
to the experienced eye they are readily detectable.

By heating a sapphire its blue colour slowly fades, to complete
transparency in many cases, or at any rate to so pale a tint as to pass
for a transparent stone. Valuable as is the sapphire, the diamond is
more so, and it follows that if one of these clear or "cleared"
sapphires is cut in the "rose" or "brilliant" form--which forms are
reserved almost exclusively for the diamond--such a stone would pass
very well as a diamond, and many so cut are sold by unscrupulous people
as the more valuable stone, which fraud an expert would, of course,
detect.

Sapphires are mentioned by Pliny, and figure largely in the ancient
history of China, Egypt, Rome, etc. The Greeks dedicated the sapphire
specially to Jupiter, and many of the stones were cut to represent the
god; it also figured as one of the chief stones worn by the Jewish High
Priest on the breast-plate. Some stones have curious rays of variegated
colour, due to their crystalline formation, taking the shape of a star;
these are called "asteriated," or "cat's eye" sapphires. Others have
curious flashes of light, technically called a "play" of light (as
described in Chapter VI. on "Colour"), together with a curious blue
opalescence; these are the "girasol." Another interesting variety of
this blue sapphire is one known as "chatoyant"; this has a rapidly
changing lustre, which seems to undulate between a green-yellow and a
luminous blue, with a phosphorescent glow, or fire, something like that
seen in the eyes of a cat in the dark, or the steady, burning glow
observed when the cat is fascinating a bird--hence its name. This is not
the same variety as the "asteriated," or "cat's eye" or "lynx eye"
mentioned above.

CHAPTER XIV.

VARIOUS PRECIOUS STONES--_continued._

_The Chrysoberyl._

There are certain stones and other minerals which, owing to their
possession of numerous microscopically fine cavities, of a globular or
tubular shape, have the appearance of "rays" or "stars," and these are
called "asteriated." Several of such stones have been discussed already
in the last chapter, and in addition to these star-like rays, some of
the stones have, running through their substance, one or more streaks,
perhaps of asbestos or calcite, some being perfectly clear, whilst
others are opalescent. When these streaks pass across the star-like
radiations they give the stone the appearance of an eye, the rays
forming the iris, the clear, opalescent, or black streak closely
resembling the slit in a cat's eye, and when these stones are cut _en
cabochon_, that is, dome-shaped (see Chapter XI. on "Cutting"), there is
nothing to deflect the light beams back and forth from facet to facet,
as in a diamond, so that the light, acting directly on these radiations
or masses of globular cavities and on the streak, causes the former to
glow like living fire, and the streak appears to vibrate, palpitate,
expand, and contract, exactly like the slit in the eye of a cat.

There are a considerable number of superstitions in connection with
these cat's-eye stones, many people regarding them as mascots, or with
disfavour, according to their colour. When possessing the favourite hue
or "fire" of the wearer, such as the fire of the opal for those born in
October, of the ruby for those born in July, etc., these stones are
considered to bring nothing but good luck; to ward off accident, danger,
and sudden death; to be a charm against being bitten by animals, and to
be a protection from poison, the "evil eye," etc. They figured largely,
along with other valuable jewels, in the worship of the ancient
Egyptians, and have been found in some of the tombs in Egypt. They also
appeared on the "systrum," which was a sacred instrument used by the
ancient Egyptians in the performance of their religious rites,
particularly in their sacrifices to the goddess Isis. This, therefore,
may be considered one of their sacred stones, whilst there is some
analogy between the cat's-eye stones and the sacred cat of the Egyptians
which recurs so often in their hieroglyphics; it is well known that our
domestic cat is not descended from the wild cat, but from the celebrated
cat of Egypt, where history records its being "domesticated" at least
thirteen centuries B.C. From there it was taken throughout Europe, where
it appeared at least a century B.C., and was kept as a pet in the homes
of the wealthy, though certain writers, speaking of the "mouse-hunters"
of the old Romans and Greeks, state that these creatures were not the
Egyptian cat, but a carniverous, long-bodied animal, after the shape of
a weasel, called "marten," of the species the "beech" or "common" marten
(_mustela foina_), found also in Britain to-day. It is also interesting
to note that the various superstitions existing with regard to the
different varieties and colours of cats also exist in an identical
manner with the corresponding colours of the minerals known as "cat's
eye."

Several varieties of cat's-eye have already been described. Another
important variety is that of the chrysoberyl called "cymophane." This is
composed of glucina, which is glucinum oxide, or beryllia, BeO, of which
there is 19.8 per cent., and alumina, or aluminium oxide, Al_{2}O_{3},
of which there is 80.2 per cent. It has, therefore, the chemical
formula, BeO,Al_{2}O_{3}. This stone shows positive electricity when
rubbed, and, unlike the sapphires described in the last chapter, which
lose their colour when heated, this variety of chrysoberyl shows no
change in colour, and any electricity given to it, either by friction or
heat, is retained for a long time. When heated in the blowpipe alone it
remains unaltered, that is, it is not fusible, and even with microcosmic
salt it requires a considerably long and fierce heat before it yields
and fuses, and acids do not act upon it. It crystallises in the 4th
(rhombic) system, and its lustre is vitreous.

The cymophane shows a number of varieties, quite as many as the
chrysoberyl, of which it is itself a variety, and these go through the
gamut of greens, from a pale white green to the stronger green of
asparagus, and through both the grey and yellow greens to dark. It is
found in Ceylon, Moravia, the Ural Mountains, Brazil, North America,
and elsewhere. The cat's-eye of this is very similar to the quartz
cat's-eye, but a comparison will make the difference so clear that they
could never be mistaken, apart from the fact that the quartz has a
specific gravity considerably lower than the chrysoberyl cat's-eye,
which latter is the true cat's-eye, and the one usually understood when
allusion is made to the stone without any distinguishing prefix, such as
the ruby, sapphire, quartz, etc., cat's eye. It should, however, be
mentioned that this stone is referred to when the names Ceylonese and
Oriental cat's-eye are given, which names are used in the trade as well
as the simple appellation, "cat's eye." One peculiarity of some of these
stones is that the "fire" or "glow" is usually altered in colour by the
colour of the light under which it is seen, the change of colour being
generally the complementary. Thus, a stone which in one light shows red,
in another will be green; the "eye" showing blue in one light will
become orange in another; whilst the yellow of another stone may show a
decided purple or amethyst in a different light.

A good test for this, and indeed most precious stones, is that they
conduct heat more quickly than does glass, and with such rapidity that
on breathing upon a stone the warmth is conducted instantly, so that,
though the stone is dimmed the dimness vanishes at once, whereas with
glass the film of moisture fades but slowly in comparison.

_The Topaz._

The name topaz is derived from the Greek _topazos_, which is the name of
a small island situated in the Gulf of Arabia, from whence the Romans
obtained a mineral which they called topazos and topazion, which mineral
to-day is termed chrysolite. The mineral topaz is found in Cornwall and
in the British Isles generally; also in Siberia, India, South America
and many other localities, some of the finest stones coming from Saxony,
Bohemia, and Brazil, especially the last-named. The cleavage is perfect
and parallel to the basal plane. It crystallises in the 4th (rhombic)
system; in lustre it is vitreous; it is transparent, or ranging from
that to translucent; the streak is white or colourless. Its colour
varies very much--some stones are straw-colour, some are grey, white,
blue, green, and orange. A very favourite colour is the pink, but in
most cases this colour is not natural to the stone, but is the result of
"burning," or "pinking" as the process is called technically, which
process is to raise the temperature of a yellow stone till the yellow
tint turns to a pink of the colour desired. The topaz is harder than
quartz, as will be seen on reference to the "Hardness" table, and is
composed of a silicate of aluminium, fluorine taking the place of some
of the oxygen. Its composition averages 16.25 per cent. of silica, 55.75
per cent. of alumina, or oxide of aluminium, and fluoride of silicium,
28 per cent. Its formula is [Al(F,OH)]_{2} SiO_{4}, or (AlF)_{2}SiO_{4}.
From this it will be understood that the fluorine will be evolved when
the stone is fused. It is, however, very difficult to fuse, and alone it
is infusible under the blowpipe, but with microcosmic salt it fuses and
evolves fluorine, and the glass of the tube in the open end of which the
stone is fixed is bitten with the gas.

Such experiments with the topaz are highly interesting, and if we take a
little of the powdered stone and mix with it a small portion of the
microcosmic salt, we may apply the usual test for analysing and proving
aluminium, thus: a strongly brilliant mass is seen when hot, and if we
moisten the powder with nitrate of cobalt and heat again, this time in
the inner flame, the mass becomes blue. Other phenomena are seen during
the influence of heat. Some stones, as stated, become pink on heating,
but if the heating is continued too long, or too strongly, the stone is
decoloured. Others, again, suffer no change, and this has led to a
slight difference of opinion amongst chemists as to whether the colour
is due to inorganic or organic matter. Heating also produces
electricity, and the stone, and even splinters of it, will give out a
curious phosphorescent light, which is sometimes yellow, sometimes blue,
or green. Friction or pressure produces strong electrification; thus the
stones may be electrified by shaking a few together in a bag, or by the
tumbling of the powdered stone-grains over each other as they roll down
a short inclined plane. The stones are usually found in the primitive
rocks, varying somewhat in different localities in their colour; many of
the Brazilian stones, when cut as diamonds, are not unlike them.

In testing, besides those qualities already enumerated, the crystalline
structure is specially perfect and unmistakable. It is doubly
refractive, whereas spinel and the diamond, which two it closely
resembles, are singly refractive. Topaz is readily electrified, and, if
perfect at terminals, becomes polarised; also the commercial solution
of violets, of which a drop only need be taken for test, is turned green
by adding to it a few grains of topaz dust, or of a little splinter
crushed to fine powder.

_The Beryl._

The beryl is a compound of silicates of beryllia and alumina, with the
formula 3BeOSiO_{2} + Al_{2}O_{3},3SiO_{2}, or
3BeO,Al_{2}O_{3},6SiO_{2}. It differs very little indeed from the
emerald, with the exception of its colour. In the ordinary varieties
this is somewhat poor, being mostly blue, or a dirty or a greenish
yellow; the better kinds, however, possess magnificent colour and
variety, such as in the aquamarine, emerald, etc. The cleavage is
parallel to the basal plane. Its lustre is sometimes resinous, sometimes
vitreous, and it crystallises in the 2nd (hexagonal) system. It occurs
in somewhat long, hexagonal prisms, with smooth, truncated planes, and
is often found in granite and the silt brought down by rivers from
granite, gneiss, and similar rocks. It is found in Great Britain and in
many parts of Europe, Asia, and America, in crystals of all sizes, from
small to the weight of several tons. The common kinds are too opaque and
colourless to be used as gems and are somewhat difficult of fusion under
the blowpipe, on the application of which heat some stones lose their
colour altogether, others partly; others, which before heating were
somewhat transparent, become clouded and opaque; others suffer no change
in colour, whilst some are improved. In almost every case a slight
fusion is seen on the sharp edges of fractures, which become smooth,
lose their sharpness, and have the appearance of partly fused glass.
The hardness varies from 7-1/4 to 8, the crystals being very brittle,
breaking with a fracture of great unevenness. The better varieties are
transparent, varying from that to translucent, and are called the
"noble" beryls. Transparent beryl crystals are used by fortune-tellers
as "gazing stones," in which they claim to see visions of future events.

_The Emerald._

Considering the particular emerald which is a variety of beryl--although
the name emerald in the trade is applied somewhat loosely to any stone
which is of the same colour, or approaching the colour of the beryl
variety--this emerald only differs chemically from the beryl, just
described, in possessing an addition of oxide of chromium. In shape,
crystallisation, fracture and hardness, it is the same, and often
contains, in addition to the chromium, the further addition of traces of
carbonate of lime, magnesia, and occasionally faint traces of hornblende
and mica, which evidently result from its intimate association with the
granite rock and gneiss, amongst which it is mostly found, the latter
rocks being of a slaty nature, in layers or plates, and, like granite,
containing mica, pyrites, felspar, quartz, etc.

Emeralds have been known from very early times, and are supposed to have
been found first in the mines of ancient Egypt. They were considered
amongst the rarest and the most costly of gems, and it was the custom,
when conferring lavish honour, to engrave or model emeralds for
presentation purposes. Thus we find Pliny describes Ptolemy giving
Lucullus, on his landing at Alexandria, an emerald on which was
engraved his portrait. Pliny also relates how the short-sighted Nero
watched the fights of gladiators through an eye-glass made of an
emerald, and in ancient times, in Rome, Greece, and Egypt, eye-glasses
made of emeralds were much valued. Many of these, as well as engraved
and carved emeralds, have been discovered in ruins and tombs of those
periods.

The copper emerald is rare; it is a hydrous form of copper silicate,
CuOSiO_{2} + H_{2}O, of a beautiful emerald green, varying from
transparent to translucent. It exhibits double refraction, and is a
crystallised mineral, brittle, and showing a green streak. This is less
hard than the real emerald, is heavier, deeper in colour, and is usually
found in crystals, in cavities of a particular kind of limestone which
exists at Altyn-Tübe, a hill in the Altai Mountains, in the Urals, and
in North and Central America.

_The Tourmaline._

The tourmaline is a most complex substance; almost every stone obtained
has a different composition, some varying but slightly, with mere traces
of certain constituents which other stones possess in a perceptible
degree. Consequently, it is not possible to give the chemical formula,
which might, and possibly would, be found but seldom, even in analyses
of many specimens. It will therefore be sufficient to state the average
composition, which is:--ferrous oxide, manganous oxide, potash, lime,
boracic acid, magnesia, soda, lithia, and water. These form, roughly
speaking, 25 per cent. of the bulk, the remainder being oxide of silicon
and oxide of aluminium in about equal parts. It crystallises in the 2nd
(hexagonal) system, with difficult cleavage and vitreous lustre.

It will naturally be expected that a substance of such complexity and
variety of composition must necessarily have a corresponding variety of
colour; thus we find in this, as in the corundum, a wonderful range of
tints. The common is the black, which is not used as a gem. Next come
the colourless specimens, which are not often cut and polished, whereas
all the transparent and coloured varieties are in great demand. To
describe adequately their characteristics with relation to light would
alone require the space of a complete volume, and the reader is referred
to the many excellent works on physics (optics) which are obtainable.
This stone is doubly refracting, exhibiting extremely strong dichroism,
especially in the blue and the green varieties. It polarises light, and
when viewed with the dichroscope shows a remarkable variety of twin
colours. It will be remembered that in Hogarth's "Rake's Progress," the
youth is too engrossed in the changing wonders of a tourmaline to notice
the entrance of the officers come to arrest him.

CHAPTER XV.

VARIOUS PRECIOUS STONES--_continued_.

_Zircon._

Zircon appears to have been first discovered by Klaproth in 1789, in the
form of an earth, and six years later he found that the stone hyacinth
contained a similar substance, both having the formula, ZrSiO_{4}, and
both having as their colouring agent ferric oxide. There are several
methods of obtaining the metallic element, zirconium; it is however with
the silicate of zirconium that we have to deal at the moment. This is
called zircon, ZrSiO_{4}, or hyacinth when transparent or red, but when
smoke-coloured, or colourless, it is the jargoon, or jarcon, and is
found in silt and alluvial soils, limestone, gneiss, and various forms
of schist, in India, Australia, the Urals, and certain parts of America.
It is often combined with and found in juxtaposition to gold and certain
varieties of precious stones. The lines of cleavage are parallel to the
sides of the prism, and the crystals have an adamantine, or diamond
lustre, varying from the completely opaque to the transparent. In some
varieties the oxide of uranium is also present in traces. It
crystallises in the 3rd (tetragonal) system, with indistinct cleavage.
Its specific gravity varies from 4.70 to 4.88, according to the specimen
and the locality.

This stone, like some of the others described, has a very wide range of
colour, going through reds, browns, greens, yellows, oranges, whites,
greys, blues from light to indigo, notwithstanding which it is somewhat
difficult to imitate scientifically, though its composition of 33 per
cent. of silica with 67 per cent. of zirconia (the oxide of zirconium),
is practically all it contains, apart from the colouring matter, such as
the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2,
consequently it is untouched by a file, and so far, if one or perhaps
two of the three qualities of colour, hardness, and specific gravity,
are obtained in a chemically made zircon, the third is wanting. Under
the blowpipe, zircons are infusible, but the coloured stones when heated
strongly become heavier, and as they are contracting, their colour
fades, sometimes entirely, which changes are permanent, so that as they
possess the adamantine lustre, they are occasionally cut like a diamond,
and used as such, though their deficiency in fire and hardness, and
their high specific gravity, make them readily distinguishable from the
diamond.

On exposure to light the coloured zircon becomes more or less
decoloured; especially is this so in sunlight, for when the direct rays
of the sun fall upon it, the colours fade, and for a moment or two
occasional phosphorescence follows, as is the case when the stone is
warmed or heated in a dark room. The stone appears to be very
susceptible to brilliant light-rays, and in certain specimens which were
split for testing, one half of each being kept excluded from light for
purposes of comparison, it was found that sunshine affected them most;
then brilliant acetylene gas, which was more effective still when tinted
yellow by being passed through yellow glass. The electric arc was not so
effective, but the electric light of the mercury-vapour lamp, though
causing little change at the first, after a few hours' exposure rapidly
bleached certain of the colours, whilst having no effect on others. Coal
gas with incandescent fibre mantle was slightly effective, whilst the
coal-gas, burned direct through an ordinary burner, affected very few of
the colours, even after twenty-four hours' exposure at a distance of
three feet. In all these cases, though the colours were slightly
improved by the stones being kept for a time in the dark, they failed to
recover their original strength, showing permanent loss of colour.

_The Silicates._

The chief of these are the garnets, crystallising in the cubic system,
and anhydrous. The garnet is usually in the form of a rhombic
dodecahedron, or as a trisoctahedron (called also sometimes an
icosatetrahedron), or a mixture of the two, though the stones appear in
other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They
average from 40 to about 42 per cent. of silica, the other ingredients
being in fairly constant and definite proportions. They are vitreous and
resinous in their lustre and of great variety of colour, chiefly amongst
reds, purples, violets, greens, yellows and blacks, according to the
colouring matter present in their mass. There are many varieties which
are named in accordance with one or more of their constituents, the best
known being: (A) The iron-alumina garnet, having the formula 6FeO,
3SiO_{2} + 2Al_{2}O_{3}, 3SiO_{2}. This is the "precious" garnet, or
almandine, sometimes called the "Oriental" garnet; these stones are
found in Great Britain, India, and South America, and are deep red and
transparent, of vitreous lustre. They get up well, but certain varieties
are so subject to defects in their substance, brought about by pressure,
volcanic action, and other causes, some of which are not yet known, that
their quality often becomes much depreciated in consequence. This
inferior variety of the iron-alumina garnet is called the "common"
garnet, and has little lustre, being sometimes opaque. The perfect
qualities, or almandine, as described above, are favourite stones with
jewellers, who mount great quantities of them.

The second variety is the (B) lime-iron garnet, formula, 6CaO,3SiO_{2} +
2Fe_{2}O_{3},3SiO_{2}. The chief of this class is the melanite,
sometimes dull, yet often vitreous; it is mostly found in volcanic
rocks, such as tuff; this variety is very popular with jewellers for
mourning ornaments, for as it is a beautiful velvet-black in colour and
quite opaque, it is pre-eminent for this purpose, being considerably
less brittle than jet, though heavier. Another variety is the
"topazolite," both yellow and green. The "aplome" is greenish-yellow,
yellowish-green, brown, and usually opaque. A further form of lime-iron
garnet is the "pyreneite," first found in the Pyrenees Mountains, hence
its name.

The (C) lime-chrome garnets--6CaO,3SiO_{2} + 2Cr_{2}O_{3}, 3SiO_{2}--the
chief of which is "uwarowite." This is of a magnificent emerald green
colour, translucent at edges and of a vitreous lustre. When heated on
the borax bead it gives an equally beautiful green, which is, however,
rather more inclined to chrome than emerald. This is an extremely rare
stone in fine colour, though cloudy and imperfect specimens are often
met with, but seldom are large stones found without flaws and of the
pure colour, which rivals that of the emerald in beauty.

The fourth variety (D) is the lime-alumina garnet, its formula
being--6CaO,3SiO_{2} + 2Al_{2}O_{3},3SiO_{2}. Like the others, it has a
number of sub-varieties, the chief being the "cinnamon stone," which is
one of great beauty and value when perfect. This stone is almost always
transparent when pure, which property is usually taken as one of the
tests of its value, for the slightest admixture or presence of other
substances cloud it, probably to opacity, in accordance with the
quantity of impurity existent. This variety is composed of the oxides of
aluminium and silicon with lime. In colour it ranges from a beautiful
yellowish-orange deepening towards the red to a pure and beautiful red.

"Romanzovite" is another beautiful variety, the colour of which ranges
through browns to black. Another important variety is the "succinite,"
which gets up well and is a favourite with jewellers because of its
beautiful, amber-like colour, without possessing any of the drawbacks of
amber.

(E) The magnesia-alumina garnet--6MgO,3SiO_{2} +
2Al_{2}O_{3},3SiO_{2}--is somewhat rare, the most frequently found being
of a strong crimson colour and transparent. This variety is called
"pyrope," the deeper and richer tints being designated "carbuncle," from
the Latin _carbunculus_, a little coal, because when this beautiful
variety of the "noble" garnet is held up between the eyes and the sun,
it is no longer a deep, blood-red, but has exactly the appearance of a
small piece of live or glowing coal, the scarlet portion of its
colour-mixture being particularly evident. The ancient Greeks called it
anthrax, which name is sometimes used in medicine to-day with reference
to the severe boil-like inflammation which, from its burning and
redness, is called a carbuncle, though it is more usual to apply the
word "anthrax" to the malignant cattle-disease which is occasionally
passed on to man by means of wool, hair, blood-clots, etc., etc., and
almost always ends fatally. A great deal of mystery and superstition has
always existed in connexion with this stone--the invisibility of the
bearer of the egg-carbuncle laid by a goldfinch, for instance.

(F) The manganese-alumina garnet--6MnO,3SiO_{2} +
2Al{2}O_{3},3SiO_{2}--is usually found in a crystalline or granular
form, and mostly in granite and in the interstices of the plates, or
laminæ, of rocks called schist. One variety of this, which is a deep
hyacinth in colour, though often of a brown-tinted red, is called
"spessartine," or "spessartite," from the district in which it is
chiefly found, though its distribution is a fairly wide one.

_The Lapis-Lazuli._

The lapis-lazuli, sometimes called "azure stone," is almost always blue,
though often containing streaks of white and gold colour, the latter of
which are due to the presence of minute specks or veins of iron pyrites,
the former and colourless streaks being due to free lime, calcite, and
other substances which have become more or less blended with the blue
colour of the stone. It has a vitreous lustre, crystallises in the 1st,
or cubic system, and is a complex substance, varying considerably in its
ingredients in accordance with the locality in which it is found, its
matrix, and the general geological formation of the surrounding
substances, which may, by the penetration of moisture, be brought to
bear upon the stone, thus influencing to a great extent its chemical
composition. So that we find the stone composed of about a quarter of
its substance of alumina, or oxide of aluminium, silica to the extent of
almost half, the remainder being lime, soda, sulphur, and occasionally
traces of copper and iron. It is mostly found in granite and certain
crystalline limestone rocks, in fairly large masses. It is of great
antiquity, figuring extensively in ancient Egyptian history, both in its
form as a stone and ground up into a pigment for the decoration of
sacred and royal vessels and appointments. When so ground, it forms the
stable and magnificent colour, _genuine_ ultramarine, which is the
finest and purest blue on the artist's palette, but owing to its
extremely high price its use is not in very great demand, especially as
many excellent chemical substitutes of equal permanence are obtainable
at little cost.

_The Turquoise._

The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour
it is blue or greenish-blue, sometimes opaque, varying between that and
feeble translucency, though it should be said that in all forms, even
those considered opaque, a thin cutting of the stone appears almost
transparent, so that the usual classing of it among the opaque stones
must be done with this reservation. In composition it contains about 20
per cent. of water, about a third of its substance being phosphoric
acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina,
with small quantities of iron in the form of variously coloured oxides,
with oxide of manganese. The great proportion of water, which it seems
to take up during formation, is mostly obtained in the cavities of
weathered and moisture-decomposing rocks. Its average formula may be
said to be Al_{2}O_{3}P_{2}O_{5} + 5H_{2}O, and sometimes Al_{2}O_{3}
FeOP_{2}O_{5} + 5H_{2}O. It must therefore follow that when the stone is
heated, this water will separate and be given off in steam, which is
found to be the case. The water comes off rapidly, the colour of the
stone altering meanwhile from its blue or blue-green to brown. If the
heat is continued sufficiently long, this brown will deepen to black,
while the flame is turned green. This is one of the tests for turquoise,
but as the stone is destroyed in the process, the experiment should be
made on a splinter from it.

This stone is of very ancient origin, and many old turquoise deposits,
now empty, have been discovered in various places. History records a
magnificent turquoise being offered in Russia for about £800 a few
centuries ago, which is a very high price for these comparatively common
stones.

Owing to the presence of phosphorus in bones, it is not uncommon to
find, in certain caves which have been the resort of wild animals, or
into which animals have fallen, that bones in time become subjected to
the oozing and moisture of their surroundings; alumina, as well as the
oxides of copper, manganese and iron, are often washed across and over
these bones lying on the cave floor, so that in time, this silt acts on
the substance of the bones, forming a variety of turquoise of exactly
the same composition as that just described, and of the same colour. So
that around the bones there eventually appears a beautiful turquoise
casing; the bone centre is also coloured like its casing, though not
entirely losing its bony characteristics, so that it really forms a kind
of ossified turquoise, surrounded by real turquoise, and this is called
the "bone turquoise" or "odontolite."

INDEX

Adamantine lustre, 28
  glimmering, 29
  glinting, or glistening, 29
  lustreless, 29
  shining, 29
  splendent, 29

Agate, 11

Almandine, 101

Amethyst, 11
  oriental, 85
  sapphire, 85

Amorphous stones and their characteristics, 23

Analysis, 5

Aplome, 101

Asters, or asteriated stones, 82, 87-91

Azure-stone, 103

Beryl, 10, 94
  colours of, in dichroscope, 34

Beryllium, 10

Bezils, 66

Black stones, list of, 79

Blue sapphire, composition of the, 10
  stones, list of, 77

Bone-turquoise, 106

Break, as opposed to cleavage, 19

Brilliant-cut stones, 66

Brown stones, list of, 76

Building up of crystals, 13

Burnt, or pinked topaz, 92

Cabochon-cut stones, 64
  (the double), 65
  (the hollow), 65

Carbonate series, 11

Carbuncle, 102, 103

Cat of Egypt, 89

Cat's eye stones, 82, 87-91
  list of (see "Chatoyant Stones"), 78

Ceylonese cat's eye (see "Cat's eye")

Change of colour (not to be confused with "Play of colour" and "Opalescence,"
which see; see also "Fire"), 36

Characteristics of precious stones, 1, 3

Chatoyant stones, list of, 78

Chemical illustration of formation of precious stones, 8

Chloride of palladium in dichroscope, 34

Chrysoberyl, 88

Chrysolite, 11
  ordinary, or "noble", 85
  oriental, 85

Cinnamon stone, 102

Claims of precious stones, 4

Cleavage affecting tests, 43
  and "cleavage" as opposed to "break", 19, 22

Colour, 26, 28, 30, 32

Colourless stones, list of, 75

Colours and characteristics of the various opals, 35, 36
  of precious stones, list of, 75-79

Common garnet, 101
  opal, 35

Composite crystals, 13

Composition of paste, or strass, for imitation stones, 71

Composition of precious stones, 7

Converted stones, 72

Corundum, 82

Crown portion of stones, 65, 66

Crystalline structure, physical properties, of 13

Crystallography, 14

Crystals, axes of symmetry, 15
  groups of, 15, 16
  planes of symmetry, 15
  systems of, 16
    (1) Cubic--isometric, monometric, regular, 16
    (2) Hexagonal--rhombohedral, 16
    (3) Tetragonal--quadratic, square prismatic, dimetric, pyramidal, 16
    (4) Rhombic--orthorhombic, prismatic, trimetric, 16
    (5) Monoclinic--clinorhombic, monosymmetric, oblique, 16, 17
    (6) Triclinic--anorthic, asymmetric, 16, 17
  treatment of, 14

Culasse portion of stones, 66

Cullinan diamond (see also "Stars of Africa"), 22, 64, 68, 80

Cutting of precious stones, 3, 4, 62

Cymophane, 90

Definition of a precious stone, 1

Diamond, characteristics of the, 80
  composition of the, 10
  (sapphire), 86
  unique, 10
  (zircon), 99

Diaphaneity, 26, 28

Diaphanous stones, 28

Dichroscope, 33
  how to make a, 33
  how to use a, 34

Dimorphism in precious stones, 25

Double cabochon-cut stones, 65
  refraction (see "Refraction")

Doublets, 72

Electric and magnetic influences, 57
  experiments with precious stones and pithball and electroscope, 57
  experiments with tourmaline, 58, 59

Emerald, 10, 11, 95, 96
  oriental, 85

En cabochon-cut stones, 64

Experiments to show electric polarity, 58, 59

Facets in stones, description of the, 67, 68

Feminine stones, 85

Fire in stones (see also "Change of Colour," "Opalescence," and "Play of
Colour"), 36, 37

Fire opal, 35

Flame-coloured stones, list of, 76

Flaws, 63

Formation of precious stones, 5, 8
  chemical illustration of, 8, 9

Garnet, 11, 100

Garnets
  (A) iron-alumina (called also almandine and precious
      or oriental garnet), 101
    sub-variety, common garnet, 101
  (B) lime-iron, 101
    sub-variety aplome, 101
      melanite, 101
      pyreneite, 101
      topazolite, 101
  (C) lime-chrome, 101, 102
    sub-variety uwarowite, 101, 102
  (D) lime-alumina, 102
    sub-variety cinnamon stone, 102
      romanzovite, 102
      succinite, 102
  (E) magnesia-alumina, 102, 103
    sub-variety carbuncle, or anthrax, 102, 103
      noble, 103
      pyrope, 102
  (F) manganese-alumina, 103
    sub-variety spessartine, or spessartite, 103

Girdle portion of a stone, 66

Glimmering, in lustre, definition of, 29

Glinting, or glistening in lustre, definition of, 29

_Goutte de suif_-cut stones, 65

Great Mogul diamond, 64

Green stones, list of, 78

Groups of crystals (see "Crystals")

Hardness, physical properties of, 39
  table of, 39, 40, 41

Heat indexes, 54
  physical properties of, 52

Hollow-cabochon, 65

Hyacinth, ordinary (a form of zircon), 85, 98
  oriental, 85

Hyalite (opal), 35

Hydrophane (opal), 35

Imitations and tests of precious stones, 70

Indigo sapphires, 86

Ink sapphires, 85

Iridescence, and cause of, 37, 38

Iron-alumina garnets, 101

Jacinth, oriental, 85

Jarcon, or jargoon, 98

Koh-i-nûr, 64

Lapis-lazuli, 103

Light, physical properties of, 26

Lime-alumina garnets, 102
  cinnamon stone, 102
  romanzovite, 102
  succinite, 102

Lime-chrome garnets, 101, 102
  uwarowite, 101, 102

Lime-iron garnets, 101
  aplome, 101
  pyreneite, 101
  topazolite, 101

List of stones according to colour, 75-79
  hardness, 39-41
  specific gravity, 48-50

Lustre, 26, 28

Lustreless, definition of, 29

Lynx-eye stones, 87

Magnesia-alumina garnets, 102, 103
  carbuncle, or anthrax, 102
  noble, 103
  pyrope, 102

Magnetic and electric influences, 57-61

Malachite, 11

Manganese-alumina garnets, 103
  spessartine, or spessartite, 103

Masculine stones, 85

Melanite, 101

Menilite (opal), 36

Metallic-lustre stones, 28, 29

Mohs's table of hardness, 39-41

Noble garnet, 103
  or precious opal, 35

Non-diaphanous stones, 28

Odontolite, 106

Olivine corundum (see "Chrysolite"), 85

Opal, 11
  varieties of, 35, 36

Opalescence (not to be confused with "Change of Colour" and "Play of Colour,"
which see; see also "Fire"), 36, 37

Oriental amethyst, 85
  cat's eye (see "Cat's eye")
  emerald, 85
  garnet, 101
  topaz, 85

Origin of precious stones, 7

Paste, or strass, for imitation stones, composition of, 71

Pavilion portion of cut stones, 66

Pearly-lustre stones, 28, 29

Peridot (see "Noble Chrysolite"), 85

Pink-coloured stones, list of (see also Red), 77

Pinked topaz, 92

Phosphorescence, 26, 30

Physical properties:--
  A.--Crystalline structure, 13
  B.--Cleavage, 19
  C.--Light, 26
  D.--Colour, 32
  E.--Hardness, 39
  F.--Specific gravity, 45
  G.--Heat, 52
  H.--Magnetic and electric influences, 57

Play of colour (not to be confused with "Change of Colour" and "Opalescence,"
which see; see also "Fire"), 36, 37

Pleochroism, 33

Polarisation, electric, 58, 59
  of light, 26, 27

Polariscope, 27, 28

Polishing precious stones, 3, 4

Polymorphism in precious stones, 25

Precious, or noble opal, 35

Pseudomorphism in precious stones, 23, 24

Pyreneite, 101

Pyro-electricity, development and behaviour of, 58-60

Pyrope, 102

Qualities of precious stones, 1, 3

Red and rose-coloured stones, list of (see also Pink), 76, 77

Reflection of light, 26, 28

Refraction of heat, 52-55
  light, 26, 27

Reproduction of crystalline form, 20, 21

Resinous lustre stones, 28, 29

Rock-crystal, 11

Romanzovite, 102

Rose-coloured stones (see Red, above), 76, 77

Rose, or rosette-cut stones, 65

Rothschild's testing solution, 73

Ruby, characteristics of, 83
  composition of, 10

Sapphire, amethyst, 85
  and its varieties, 84, 85
  cleared, 86
  diamonds, 87
  indigo, 86
  ink, 85
  the blue, composition of, 10, 85
  water, 86

Semi-diaphanous stones, 28

Shining, in lustre, definition of, 29

Silica group, composition of the, 11

Silicates, 100

Silky-lustre stones, 28, 29

Single-refraction (see "Refraction")

South African diamond (see "Cullinan Diamond")

Specific gravity, 45

Splendent, in lustre, definition of, 29

Splitting of the Cullinan diamond, 22

Star-portion of stones, 65

Stars of Africa (see also "Cullinan Diamond"), 22, 64, 68

Starting or splitting of stones on cleavage planes, 23

Step-cut stones, 66

Stones arranged according to colour, 75-79
  hardness, 39-41
  specific gravity, 48-50

Strass for imitation stones, composition of, 71

Sub-metallic in lustre, definition of, 29

Sub-translucent stones, 28

Sub-transparent stones, 28

Succinite, 102

Synthesis, 5

Systems of crystals (see "Crystals")

Table-cut stones, 65

Tallow drops, 65

Teeth of stone, 65

Testing by crystalline structure, 17
  hardness, 40, 43
    with needles, 41
  gems by dichroscope, 33, 34
  solution (Rothschild's), 73

Tests of precious stones (general), 70

Topaz, 11, 91
  colours of, in dichroscope, 34
  oriental, 85

Topazolite, 101

Tourmaline, 96, 97
  electric experiments with, 58, 59

Translucent stones, 28

Transmission of heat, 52-56
  light, 26

Transparent stones, 28

Trap-cut stones, 66

Tri-morphism in precious stones, 25

Triplets, 72

Turquoise, 104
  (bone), 106
  composition of the, 11
  odontolite, 106

Uwarowite, 101, 102

Violet stones, list of, 78

Vitreous-lustre stones, 28, 29

Water-sapphires, 86

White (paste) stones, 71
  stones, list of, 75

Yellow stones, list of, 76
  topaz, 92

Zircon, 10, 98
  diamonds, 99

Zirconium, 10

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