LAPIS LAZULI, a mineral substance valued for decora tive purposes in consequence of its deep blue colour. It is opaque and takes a good polish, and since ancient times it has been much used for small ornaments and for inlaying. Its beautiful colour led to its employment, when ground and levigated, as ultrama rine (q.v.), a valuable pigment now displaced by a chemical product (artificial ultramarine). Lapis lazuli occurs as compact masses, with finely granular structure, its specific gravity is 2.38 to 2.45, and its hardness about 5.5, so that being comparatively soft it tends, when polished, to lose its lustre rather readily. Chemical analyses show considerable variation in composition, and thin sections under the microscope show blue particles and various minerals embedded in a white matrix. The main constitu ents are minerals of the sodalite group, principally lazulite, which has chemical formula: and is occasion ally found as deep blue crystals of rhombic-docecahedron form. In addition to the blue cubic minerals in lapis lazuli the follow ing have also been found : diopside, amphibole, felspars, mica, apatite, sphene, zircon, calcite and pyrite.
Lapis lazuli usually occurs in crystalline limestone, and is a product of contact metamorphism near granite masses. The best known and probably the most important locality is in Badak shan, Afghanistan, where it occurs in limestone, in the valley of the Kokcha, a tributary to the Oxus south of Firgamu. The mines were visited by Marco Polo in 1271. Another important locality is in Siberia near the western end of Lake Baikal. Fine masses of paler blue lapis lazuli occur in the Andes, near Ovalle, Chile. In Europe it is found as a rarity in the peperino of Latium, near Rome, and in the ejected blocks of metamorphosed limestone on Monte Somma, Vesuvius. Much of the material now sold as "lapis" is an artificially coloured hornstone or jasper from Ger many. This shows colourless specks of clear crystallized quartz; and never the gold-like specks of pyrite, so characteristic of true lapis lazuli, and fancifully compared with stars in the deep blue firmament. (L. J. S.)