We seldom get specimens of pure calcite as it often occurs in association with other mineral specimens. It is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The other polymorphs are the aragonite and vaterite. Aragonite will change to calcite at 380–470 °C and vaterite is even less stable. It has a hardness of 3 on the Mohs scale, its specific gravity is 2.71 and its luster is vitreous in crystallized varieties. The color is most commonly white or clear, but can appear gray, red, orange, yellow, green, blue, violet, brown, or even black due to the presence of various impurities.