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 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.
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.