Coral is a mineral with a hardness of 3 out of 10 on the Mohs scale of mineral hardness [?]. These Trigonally structured gems are made of calcium carbon trioxide, their full chemical compound being CaCO3(or C3H48N9O11).
Corals are the supporting framework of small polyps. Each coral polyp, a tiny marine animal that lives in enormous colonies, extracts calcium carbonate from the sea and exudes it through their bases to build a protective home around and above itself. Each generation of polyps dies in its protective home and each succeeding generation builds on top of its predecessor. They precipitate calcareous material in their outer body wall and thus build reefs, atolls, and coral banks with many intertwining branches down to a depth of 300 meters. Only these branches are used for working as precious stones. Because of its appearance, coral is also called "scorpion stone."
Corals are thus composed of the remains of millions of very small coral polyps in the form of colonies.
Coral is harvested in a weighted, wide-meshed net dredged across the sea bed. But because coral grows with its broad base on the rocky seabed, this method destroys much valuable material. When it is brought to the surface, the soft parts are rubbed away and the material sorted according to quality. Today, measures are being taken to regulate trade and protect coral habitats, and sustainable harvesting continues to be developed and applied by state-licensed divers. Coral farms are a source of coral where no harm is done to the reef.
In the unworked state, coral is dull or has a waxy luster, but when polished has a vitreous luster. It is usually found on the coasts of the Mediterranean, Bay of Biscay, Canary islands, Midway Islands (Japan).
Bamboo coral, a commercial term for coral suitable for jewelry, is fished from the waters of Tasmania, Australia and has a structure similar to bamboo.
Natural gem corals are found in the Mediterranean Sea, Persian Gulf, Australia, Japan, Malaysia, Korea, Hawaii, China, and Ireland. Gem coral ranges from semitranslucent to opaque and occurs in white, pink, orange, red, blue, violet, golden, and black. A pale, rose-colored coral from Japan is called "boke."
The most valued type of coral is the noble red coral (Corallicum rubrum). The color is uniform throughout the branch, from soft pink to dark red, sometimes white or pink spotted. The red color is caused by iron oxide or may be organic partly or both. "Arciscuro" or "Carbonetto" is an Italian color grade classification for very dark red coral. "Blood coral" is a term used for intense precious red coral. Other red classifications include "Pelle d'angelo" which is rose-red, "Rose pallido" a pale rose color coral, "Rosa vivo" a bright rose, "Secondo coloro," which is salmon rose, and "Rosso scuro," a dark red.
White, black, and blue coral are also used. White coral as well as red is composed of calcium carbonate. "Bianco" is the Italian classification for white coral of gem quality, valued in the Orient.
Most black and golden corals are largely horny organic substances, not calcium carbonate. Some black and dark blue corals are formed mainly of calcite or conchiolin, and are found in the Malaysian archipelago, Red Sea, and northern Australia.
Black coral grows to a height of 3 meters. It is also known as "accarbaar," "akabar,""King's coral" (Antipathes spiralis), and is the highest trade-quality black coral in SE Asia. The lowest quality of precious coral is called "bruciato" or "burnt coral" which has discolored naturally, at the bottom of the sea.
Medium-quality coral is also sourced from the coast of Algeria.
It is sensitive to heat, acids, and hot solutions, and its color can fade when worn.
Coral takes its name from the Greek. Blue coral (akori coral) Allopori subirolcea, was collected, fashioned, and prized by the people of the West African coast and in Samoa, although the name "akori" has recently been applied to substitutes like pearl, glass, white coral, and rock.
Coral is an organic gem material. Only their calcified skeletons are used in jewelry. The finest coral is used to make figurines, cameos, carvings, and beads. Little stick-like rods are drilled and strung as spiky necklaces. From agatized/fossil corals, cabochons, ornamental objects and sculptures are also produced, sometimes dyed blue or pink.
It is worked with saw, knife, file and drill, and is rarely cut and polished. It is sometimes used as an imitation for pink conch pearl.
The main trade center is Torre del Greco, south of Naples (Italy). Corals are also imported from Japan, Australia, and Hawaii.
There are many imitations made from glass, horn, rubber, bone, plastics, coral red glasses, stained or not stained coral dust cemented together, porcelain, dyed shell, stained calcined bone, stained vegetable ivory, powdered gypsum or limestone or rubber mixed with isinglass and cinnabar or lead oxide. Such colored stones can be discriminated by wiping the surface with a nail-polish moistened cotton ball, which removes amyl acetate. Cinnabar is used as a coloring agent in the production of imitation coral. The Gilson company in France makes a coral imitation in various shades of reddish pink. The chief constituent is calcite from a French source, but some ingredients not found in natural coral are present.