Azurite is a secondary mineral, a hydrated copper carbonate mineral, chemical formula (Cu3(CO3)2(OH)2. It occurs both in crystalline form (monocline) and as an amorphous precipitate. Its name refers to its color, a deep blue azure-blue that makes it popular as a collector's item, but only rarely used in jewelry on account of its susceptibility to wear and color loss. In medieval times, the mineral was ground up and used by painters as a blue pigment. The azurite in this sample has been precipitated in a nodular concretion that has been cut in half, of which both faces have been polished.