This variety is iron-aluminium garnet corresponding to the formula Fe3Al2(SiO4)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 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 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 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.