Properties of Lapis Lazuli
Rock made primarily of lazurite (Na, Ca)8(Al, Si)12O24(S, SO4). Also contains hauyne, sodalite and nosean, which are all members of the sodalite group.
Hardness (Mohs) Variable. Generally 5–6
Specific Gravity Variable. Generally 2.7–2.9
Refractive Index ca. 1.50
Crystal System None (lapis is a rock). Lazurite, the main constituent, is isometric, and frequently occurs as dodecahedra.
Colors Blue, mottled with white calcite and brassy pyrite
Handling Due to its softness, care must be taken in the wearing of lapis lazuli.
Enhancements Frequently dyed or impregnated
Synthetic available? Yes
Lapis lazuli is a semiprecious stone, valued for its deep blue colour. The source of the pigment ultramarine, it is not a mineral but a rock colored by lazurite (see sodalite). In addition to the sodalite minerals in lapis lazuli, small amounts of white calcite and of pyrite crystals are usually present. Diopside, amphibole, feldspar, mica, apatite, titanite (sphene), and zircon may also occur.
Because lapis is a rock of varying composition, its physical properties are variable. It usually occurs in crystalline limestones and is a product of contact metamorphism. The most important sources are the mines in Badakhshan, northeastern Afghanistan, and those near Ovalle, Chile, where it is usually pale rather than deep blue. Much of the material that is sold as lapis is an artificially colored jasper from Germany that shows colourless specks of clear, crystallized quartz and never the gold like flecks of pyrite that are characteristic of lapis lazuli and have been compared with stars in the sky.