Chemical composition -- An organic gem material composed of calcium carbonate (calcite). Coral is the mass of skeletons of a colony of tiny marine polyps.
Color -- Pink, red, white, black. Dark red coral is referred to as oxblood; light pink, as angelskin.
Optics -- R.I. 1.49-1.69.
Durability -- Hardness 3.5-4. Soft enough to be worked with ordinary metal cutting tools (avoid excessive heat). Although soft, coral is tough enough to be worn in jewelry. Its most famous use is in jewelry from Italy and the Native American tribes of the southwest.
Crystal structure -- Hexagonal(massive). Typically occurs in treelike, branched structures.
Specific Gravity -- 2.6-2.7, except for black coral (1.34).
Sources -- The Mediterranean Sea, Japan, and the South China Sea are the best known locations. Black coral is found in Hawaii.