Lapis

Lapis lazuli is a rock, not a mineral: whereas a mineral has only one constituent, lapis lazuli is formed from more than one mineral.^[2]

The main component of lapis lazuli is lazurite (25% to 40%), a feldspathoid silicate mineral composed of sodium, aluminium, silicon, oxygen, sulfur, and chloride. Its formula is (Na,Ca)₈(AlSiO₄)₆(S,SO₄,Cl)_1-2.^[3] Most lapis lazuli also contains calcite (white), sodalite (blue) and pyrite (metallic yellow). Other possible constituents are augite, diopside, enstatite, mica, hauynite, hornblende and nosean. Some contain trace amounts of the sulfur rich lollingite variety geyerite.

Lapis lazuli usually occurs in crystalline marble as a result of contact metamorphism.

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 no calcite at all or too much 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 this is often a very dark blue with a noticeable grey cast, may also appear as a milky shade.