Lapis lazuli is an intensely blue stone made of a deep blue lazurite, cloudy white calcite, sparkly pyrite and other minerals. This beautiful mixture of textures gives lapis lazuli its characteristic look.
Lapis lazuli is found with limestone or as separate boulders. Stones from what is now modern Afghanistan and Pakistan are still considered to be of the best quality. Argentina also produces a premium deep blue variety.
Lapis lazuli is found in the USA and Canada and less intense blue examples are found in Chile and in the former USSR.
Lapis lazuli is most widely known for its use as a gemstone. It is a popular material for cutting into cabochons and beads. It is also used in inlay or mosaic projects and often as a material for small sculptures. These uses made lapis the most popular opaque blue gemstone.
Although personal preferences vary, the most popular lapis has a uniform, deep blue to violet blue color. Many people enjoy a few randomly placed grains of gold pyrite or a few fractures or mottles of white calcite. However, when pyrite or calcite is present in more than minor amounts, the desirability of the material and the value are significantly lowered. Gray inclusions or mottling also quickly lowers desirability.
Sources: Firemountain Gems; Geology.com