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 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 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.
CHAPTER XXIV
TOURMALINE
(Rubellite)
TOURMALINE is unsurpassed even by corundum in variety of hue, and it has during recent years rapidly advanced in public favour, mainly owing to the prodigal profusion in which nature has formed it in that favoured State, California, the garden of the west. Its comparative softness militates against its use in rings, but its gorgeous coloration renders it admirably fitted for service in any article of jewellery, such as a brooch or a pendant, in which a large central stone is required. Like all coloured stones it is generally brilliant-cut in front and step-cut at the back, but occasionally it is sufficiently fibrous in structure to display, when cut en cabochon, pronounced chatoyancy.
The composition of this complex species has long been a vexed question among mineralogists, but considerable light was recently thrown on the subject by Schaller, who showed that all varieties of tourmaline may be referred to a formula of the type 12SiO2.3B2O3.(9-x)[(Al,Fe)2O3].3x[(Fe,Mn,Ca,Mg,K2,Na2,Li2,H2)O].3H2O. The ratios of boric oxide, silica, and water are nearly constant in all analyses, but great variation is possible in the proportions of the other constituents. Having regard to this complexity, it is not surprising to find that the range in colour is so great. Colourless stones, to which the name achroite is sometimes given, were at one time exceedingly rare, but they are now found in greater number in California. Stones which are most suited to jewellery purposes are comparatively free from iron, and apparently owe their wonderful tints to the alkaline earths; lithia, for instance, is responsible for the beautiful tint of the highly prized rubellite, and magnesia, no doubt, for the colour of the brown stones of various tints. Tourmaline rich in iron is black and almost opaque. It is a striking peculiarity of the species that the crystals are rarely uniform in colour throughout, the boundaries between the differently coloured portions being sharp and abrupt, and the tints remarkably in contrast. Sometimes the sections are separated by planes at right angles to the length of the crystal, and sometimes they are zonal, bounded by cylindrical surfaces running parallel to the same length. In the latter case a section perpendicular to the length shows zones of at least three contrasting tints. In the Brazilian stones the core is generally red, bounded by white, with green on the exterior, while the reverse is the case in the Californian stones, the core being green or yellow, bounded by white, with red on the exterior. Tourmaline may, indeed, be found of almost every imaginable tint, except, perhaps, the emerald green and the royal sapphire-blue. The principal varieties are rose-red and pink (rubellite) ([Plate XXVII], Fig. 1), green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian sapphire), yellowish green (Brazilian peridot) ([Plate XXVII], Fig. 2), honey-yellow (Ceylonese peridot), violet-red (siberite), and brown ([Plate XXVII], Fig. 8). The black, opaque stones are termed schorl.
The name of the species is derived from the Ceylonese word, turamali, and was first employed when a parcel of gem-stones was brought to Amsterdam from Ceylon in 1703; in Ceylon, however, the term is applied by native jewellers to the yellow zircon commonly found in the island. Schorl, the derivation of which is unknown, is the ancient name for the species, and is still used in that sense by miners, but it has been restricted by science to the black variety. The ‘Brazilian emerald’ was introduced into Europe in the seventeenth century and was not favourably received, possibly because the stones were too dark in colour and were not properly cut; that they should have been confused with the true emerald is eloquent testimony to the extreme ignorance of the characters of gem-stones prevalent in those dark ages. Achroite comes from the Greek, ἄχροος, without colour.