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 When ground to powder it furnishes a fine blue paint, but it has now been entirely superseded for this purpose by an artificial product.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, Na_{4}(NaS_{3}.Al)Al_{2}Si_{3}O_{12}, has specific gravity varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite, (Na_{2},Ca)_{2}(NaSO_{4},Al)Al_{2}Si_{3}O_{12}, is about the same in specific gravity, 2·4 to 2·5, but slightly harder, 5½ to 6; while sodalite, Na_{4}(AlCl)Al_{2}Si_{3}O_{12}, 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, MnSiO_{3}, 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.