Precious coral is a gemstone quality variety of natural coral, closely related to reef-building 'stony' coral. Precious coral is a deep water coral that forms in rocky seabeds with low levels of sedimentation, typically in dark environments of over 500 feet deep, including caverns and crevices. Precious coral is slowly built by very small marine animals known as coral polyps. These tiny, soft-bodied creatures form with minute, hard shells that accumulate as the colony grows. Over time, the colony begins to form complex branches and skeletal structures composed of hardened calcium carbonate, and colored by carotenoid pigments. These biological formations grow very slowly, often as little as just 1 millimeter per year, ranging in size from small, hand-sized structures to enormous coral reefs.