Red coral is largely calcium carbonate. Fine quality coral is prized for its colour and vitreous luster, with bright red Mediterrarean coral being the most valuable. Coral has a very low hardness of around 3. Its constituent calcite crystals form in the trigonal system, however the proteinaceous deposits of some coral are considered non-crystalline. Coral generally has a specific gravity of around 2.65, yeat can be as low as 2.35 when containing more organic compounds. Blue coral is lower at 2.46. Its refractive indices are 1.49/1.66.