Tourmaline

Tourmaline, a mineral which has a very complex and somewhat variable chemical composition. The chief constituents are silica and alumina in nearly equal proportions, and forming about three-fourths of the whole. The remainder consists of boracic acid, ferrous oxide, manganese oxide, magnesia, lime, soda, potash, and lithia, which are not all present, however, in any specimen. Tourmaline has a hardness = 7 to 7.5, and is thus harder than quartz, but not so hard as topaz. Its specific gravity is 2.94 to 3.3. It crystallises in rhombohedral forms, which are usually hemihedral, the prisms being often triangular or six-sided, and variously acuminated. The mineral has a vitreous lustre, and varies from transparent to opaque. The most common colours are black, brownish black, and bluish black; blue, green, and red varieties also occur; but white or colourless kinds are rare. Red tourmaline is known as rubellite; pale blue or bluish black as indicolite; Berlin-blue and transparent as Brazilian sapphire (in jewellery); green and transparent as Brazilian emerald, chrysolite, or peridot of Brazil; honey-yellow as peridot of Ceylon; colourless as achroite; black as schorl. Tourmaline occurs frequently in drusy cavities in granite, and also as an accessory mineral in granite, gneiss, and many crystalline schists, as also in certain granular dolomites and crystalline limestones associated with schists and plutonic rocks. Not infrequently it occurs along with various precious stones in the sands and alluvia derived from the disintegration of plutonic rocks and crystalline schists, as in Ceylon, Siberia, and Brazil. Various kinds are found in the Shan districts of Burma. The finest tourmalines are much valued by jewellers, but are comparatively rare. The black variety, schorl, is common in many of the granites and schists of Britain.