Calcite is a carbonate mineral and is the most stable form of calcium carbonate. With a hardness of only 3 on the Mohs scale, it can only be used in jewelry that won't be subject to knocks or scratches, such as earrings and pendants. Its hardness is similar to that of gems like amber, coral and chrysocolla. Though calcite is not an especially durable material, it does take an excellent polish. Many historical artifacts identified as alabaster are in fact made from calcite.
Pure calcium carbonate is colorless, but calcite is often colored by various impurities, including iron, magnesium, manganese, zinc or cobalt. Thus you will find calcite in a range of colors including white, gray, yellow, green, red and blue.