The lovely blue stone known as lapis lazuli has since the earliest times been applied to all kinds of decorative purposes, for mosaic and inlaid work and as the material for vases, boxes, and so on, and was the original sapphire of the ancients. When ground to powder it furnishes a fine blue paint, but it has now been entirely superseded for this purpose by an artificial product. Although to the eye so homogeneous and uniform in structure, lapis lazuli has been shown by microscopic examination to be composed of calcite coloured by three blue minerals in varying proportions. All three belong to the cubic class of symmetry, and are mainly soda aluminium silicates in composition; their hardness varies from 5 to 6 on Mohs’s scale. Lazurite, Na4(NaS3.Al)Al2Si3O12, has specific gravity varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite, (Na2,Ca)2(NaSO4,Al)Al2Si3O12, is about the same in specific gravity, 2·4 to 2·5, but slightly harder, 5½ to 6; while sodalite, Na4(AlCl)Al2Si3O12, is the lightest in density, 2·14 to 2·30, with hardness 5½ to 6, and has a refractive index 1·483.
By far the oldest mines are in the Badakshan district of Afghanistan, a few miles above Firgamu in the valley of the Kokcha, a branch of the Oxus, where ruby and spinel are found. It is also found at the southern end of Lake Baikal, Siberia, and in the Chilian Andes.
Sodalite occurs in beautiful blue masses at Dungannon, Hastings County, Ontario, Canada, and at Litchfield, Maine, U.S.A. They make excellent polished stones.
Violane, a massive, dark violet-blue diopside from San Marcel, Piedmont, Italy, also makes a handsome polished stone.
Rhodonite, silicate of manganese, MnSiO3, possesses a fine red colour, and makes an attractive stone when cut and polished. It has very slight biaxial double refraction, the refractivity being about 1·73; the specific gravity is 3·6, and hardness 6. It is found in large masses near Ekaterinburg in the Ural Mountains, and is quarried as an ornamental stone.
Both the copper carbonates, azurite or chessylite, and malachite, make effective polished stones. The latter is also worked into various ornamental objects; it occurs in fibrous masses, the grained character of which look well in the polished section. Its colour is a bright green, to which it owes its name, from μαλακή, mallows. Its composition is represented by the formula CuCO3.Cu(OH)2, and it is the more stable form, since azurite is frequently found altered to it. It has biaxial double refraction, and the indices are about 1·88; the specific gravity is 4·01, and hardness about 3½ to 4 on Mohs’s scale. It is found in large masses at the copper mines of Nizhni Tagilsk in the Ural Mountains, where it is mined as an ornamental stone; it also accompanies the copper ores in many parts of the world, for instance Cuba, Chili, and Australia. Azurite, so called on account of its beautiful blue colour, is rarer, but, unlike malachite, is generally in the form of crystals. Beautiful specimens have come from Chessy, near Lyons, France, and Bisbee, Arizona, U.S.A. The composition corresponds to the formula 2CuCO3, Cu(OH)2. The specific gravity is 3·80, and hardness about 3½ to 4.
Chrysocolla occurs in blue and bluish-green earthy masses, with an enamel-like texture, which in some instances can be worked and polished. Being the result of the decomposition of copper ores, it varies considerably in hardness, ranging from 2 to 4 on Mohs’s scale. Its composition approaches to the formula CuSiO3.2H2O, but it invariably contains impurities. It is very light, the density being only about 2·2.
Steatite, or soapstone, is a massive foliated silicate of magnesium corresponding to the formula H2Mg3Si4O12, which is one of the softest of mineral substances, representing the degree 1 on Mohs’s scale, but in massive pieces is harder owing to the intermixture of other substances with it. It has a peculiar greasy feeling to the touch, due to its softness. The specific gravity is about 2·75. The Chinese carve images out of the yellowish and brownish pieces.
Meerschaum, a silicate of magnesium corresponding to the formula H4Mg2Si3O10, is familiar to every smoker as a material for pipe-bowls. It is very light, the specific gravity being only 2·0, and soft, the hardness being about 2 to 2½ on Mohs’s scale. When found, it is pure white in colour, and answers to its name, a German word signifying sea-foam. It comes from Asia Minor.
Serpentine has been largely used for decorative purposes, as well as for cameos and intaglios, and formed most of the famous ‘verde antique.’ Being the result of the decomposition of other silicates it varies enormously in appearance and characters, but the most attractive stones are a rich oil-green in colour and resemble jade. The composition approximates to the formula H4Mg3Si2O9, but it invariably contains other elements. The hardness varies from 2½ to 4 on Mohs’s scale, according to the minerals contained in the stone; the specific gravity is about 2·60 and the refractivity 1·570.