The main component of lapis lazuli is lazurite (25 to 40 percent), a feldspathoid silicate mineral composed of sodium, aluminium, silicon, oxygen, sulfur, and chlorine. Most lapis also contains calcite (white), sodalite (blue) and pyrite (yellow). Other possible constituents are augite, diopside, enstatite, mica, hauynite, hornblende and nosean. Lazurite's formula is (Na,Ca)8(AlSiO4)6(S,SO4,Cl)1-2.
Lapis lazuli usually occurs in crystalline limestones as a result of contact metamorphism.
Inclusions are common in lapis and are an important help in identifying the stone. The finest color is intense blue, lightly dusted with small flecks of golden pyrite. There should be no white calcite veins and the pyrite inclusions should be small. Stones that contain too much calcite or pyrite are not as valuable. Patches of pyrite are an important help in identifying the stone as genuine and do not detract from its value. Often, inferior lapis is dyed to improve its color, but these are often a very dark blue with a noticeable grey cast.
The finest lapis comes from the Badakshan area of Afghanistan. This source of lapis may be the oldest continually worked set of mines in the world, the same mines operating today having supplied the lapis of the pharaohs. More recently, during the 1980s conflict with the USSR, Afghani resistance fighters disassembled unexploded Soviet landmines and ordnance and used the scavenged explosive to help mine lapis to further fund their resistance efforts.
In addition to the Afghan deposits, lapis has been found in the Andes near Ovalle, Chile, where it is usually pale rather than deep blue. Other less important sources are the Lake Baikal region of Russia, Siberia, Angola, Burma, Pakistan, USA (California and Colorado) and Canada.
Edited from Wikipedia
Image by John H. Betts