Common worldwide, calcite is the principal component of limestone's and marbles, and of most stalactites and stalagmites. It can also he found as large, transparent, colorless, complex crystals, or as prismatic crystals inter rowan with other minerals. because of its softness it is only faceted for the collector, hut marbles and brown, handed calcite from limestone caves are both used for decoration and carving.

Italy is famous for fine quality marbles, particularly the creamy Carrara marble. Transparent, colorless rhombs are known as "Iceland spar"; a white Fibrous variety, cut en Cabochon, shows the cat's-eye effect. Pink and green crystals occur in the USA, Germany, and England.

Much of the smoky quartz on the market is in fact irradiated rock crystal. Brown quartz has been confused with andalusite, axinite, indorse. And brown tourmaline.