Gems - Garnet
( Originally Published 1919 )
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 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 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 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 seldom homogeneous, being usually composed of several distinct individuals of a lower order of symmetry. Al-though 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 1740 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 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 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 61 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 1s. 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.
(Grossular, Cinnamon-Stone, Hyacinth, Jacinth)
This variety, strictly a calcium-aluminium garnet corresponding to the formula Ca3Al2(SiO4)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 noowv 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 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 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 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.
(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 Mg3Al2(SiO4)3, but usually contains some ferrous oxide and thus approaches almandine. Both are included among the precious garnets. Its name is derived from , 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 7 1/4.
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 468 carats, set in an Order of the Golden Fleece.
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 1/4. It exhibits in the spectroscope the absorption-bands characteristic of almandine.
This variety is iron-aluminium garnet corresponding to the formula Fe3AI2(SiO4)3, but the composition is very variable. In colour it is deep crimson and violet or columbine-red (Plate )(XIX, 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 trans-parent 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 7 1/2 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.
Properly a manganese-aluminium garnet corresponding to the formula Mn3Al2(SiO4)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, indeed, been found in Ceylon (p1 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 1.8 1, 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 Ca3Fe2(SiO4)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 6 1/2 on Mohs's scale.
This variety, which is altogether unknown in jewellery, is a calcium-iron garnet corresponding mainly to the formula Ca3Cr2(SiO4)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 7 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.