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 prevents its attaining any popularity with those professing
a nice taste in gem-stones. It must not, however, be supposed that
garnet has entirely disappeared 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
impossibility of distinguishing these two species in 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 originally 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 distressed 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 conform 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 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.
[Illustration: FIGS. 75, 76.—Garnet Crystals.]
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 seldom homogeneous, being usually composed of
several distinct individuals of a lower order of symmetry. Although
singly refractive as far as can be determined with the refractometer 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 prolonged 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 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 1·740 and 1·890. Both the
specific gravity and the refractive index increase on the whole with
the percentage amount of iron.
Garnet is a prominent constituent 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 resulting 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 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 6¾ 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
10s. 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_, _Jacinth_)
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 therefore 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 ἥσσων
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 indiscriminately 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 transparent
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 characteristic, 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 1·742 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.
(_b_) PYROPE
(‘_Cape-Ruby_’)
Often quite ruby-red in colour (Plate XXIX, Fig. 6), this variety
is probably the most popular of 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 πυρωπός, 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 1·740; 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
7¼.
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. According to De Boodt the Kaiser Rudolph II had
one in his possession valued at 45,000 thalers (about £6750). The
Imperial Treasury at Vienna contains 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 468½ 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 otherwise
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 1·760, and hardness 7¼. It exhibits in the spectroscope the
absorption-bands characteristic of almandine.
(_d_) ALMANDINE
(_Carbuncle_)
This variety is iron-aluminium garnet corresponding to the formula
Fe_{3}Al_{2}(SiO_{4})_{3}, but the composition 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 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 enriching 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 transparent stones to 4·20
for the densest black stones, and the refractive index may be as high
as 1·810. Almandine is one of the hardest of the garnets, and is
represented by the symbol 7½ 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 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 corresponding 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 acceptable in jewellery. Two
splendid stones have, indeed, been found in Ceylon (p. 211), and good
stones rather resembling hessonites have been quarried at Amelia
Court House in Virginia, and others have come from Nevada; otherwise,
spessartite is unknown as a gem-stone.
The specific gravity ranges from 4·0 to 4·3, and the refractive index
is about 1·81, both characters being high; the hardness is slightly
greater than that of quartz.
(_f_) ANDRADITE
(_Demantoid_, _Topazolite_, ‘_Olivine_’)
Andradite is strictly a calcium-iron garnet corresponding 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.
Once contemptuously styled common garnet, andradite 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 challenging 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 about 3·85 for
demantoid, which has a high refractive index, varying from 1·880 to
1·890, and may with advantage be cut in the brilliant form. It is the
softest of the garnets, being only 6½ on Mohs’s scale.
(_g_) UVAROVITE
This variety, which is altogether unknown in jewellery, is
a calcium-iron garnet corresponding 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 3·41 to 3·52. Unlike the kindred varieties it
cannot be fused by heating before an ordinary blowpipe.