Coral polyps are marine plant-like animals and are extremely small, barely 2 millimeters in diameter. Coral’s external skeletons mass together to form tree like structures. They live in colonies, either in reef systems in warm, shallow water or in deep sea colonies. Coral polyps have a hollow cylindrical body, with a ring of tentacles around the mouth, and secrete an external chalky or horny skeleton. The coral grows as the polyps ingest small animals and plants, and secrete more of the skeleton-forming substances. Red, pink, white, and blue coral are made of calcium carbonate, while black and golden corals are formed of a horny substance called conchiolin. Coral has a dull luster when recovered, but can take a bright polish.

There are two thousand types of coral in existence. Coral is found at depths of 1-1020 ft (3-300 m), and mainly harvested with weighted, wide-meshed nets dredged across the seabed. When harvested by divers, not as many corals are damaged. Reef coral (including precious noble coral as well as blue, golden and black coral) and deep sea corals are the most threatened, primarily by the fishing industry, the rise in water temperature from global warming and deep sea oil and gas development.

There are different types of coral: sponge coral (often attached to reefs, it is not rare and inexpensive), it occurs naturally in reds, purples and yellows but is also dyed or stained; noble coral (also known as precious noble coral occurs both in deep sea colonies and in coral reefs with warm shallow water), it is very hard to polish which adds to its expensive price in the market. There are different types of noble coral: momo coral (beautiful light red to salmon color, often mottled with white areas); sardegna coral (very popular for its medium red color, if you see it in a bright red color it is dyed); angel skin coral (a pretty rose-pink color with whitish or light red spots – popular with jewelry makers); moro coral (has a darker, oxblood color – it is the rarest and most expensive type of noble coral and comes from the seas around Italy – very rare due to the damage caused to the reefs by overfishing and pollution); and Japanese red coral (red with white patches). There is also black coral (not a very important type – inexpensive – it is thought to be coral in an early state of decay); golden coral (like the black coral they consist of the hornlike substance conchiolin – found in the coastal waters of Hawaii and has a resinous or lacquered luster and texture); and blue coral (has very little value as a stone – blue coral is thought to be in the first stage of decay).

For the last 200 years, the main trade center for coral has been Torre del Greco, in the south of Naples, Italy. Three quarters of the coral harvested all over the world are still processed here. Deposits of coral are found along the coast of the western Mediterranean countries, the Red Sea, Bay of Biscay, Canary Islands, Malaysian Archipelago, the Midway Islands, Japan, and Hawaii in the United States. Production is increasingly controlled by environmental laws.

Coral can be confused with conch pearl, carnelian, rhodonite, and spessartite. Imitations are made from glass, horn, bone, rubber and plastics.

Information gathered from