Calcite

Calcite is calcium carbonate, CaCO₃, and takes more crystal forms than almost any other mineral: rhombohedra, pointed scalenohedra known as dogtooth spar, long prisms and flat nailhead crystals. Pure calcite is colorless or white; impurities make it yellow, orange, green, blue or pink, the pink of Katanga coming from cobalt. Clear calcite splits light into two rays, so that a line seen through it appears doubled. The classic material for this was Iceland spar from Helgustaðir in eastern Iceland. Collectors know calcite from St. Andreasberg in the Harz, the iron mines of Cumbria, the Elmwood Mine in Tennessee, Dalnegorsk in Russia and the tin mines around Chenzhou in Hunan. Calcite is soft and dissolves even in weak acid, so it is never cleaned with acid.