Information on Corals

by Manisha G

coral-stoneCoral can be described as the collective skeletons of a vast number of the curious little organisms known as Coral Polyps. These begin life as tiny free-swimming very primi­tive organisms living in the warm waters of the sea at depths of 10-500 feet or so. These eventually attach themselves to a previous coralline growth where they form part of a vast colony in which the individuals share a common gelatinous base known as the coenosarc. It is from a membrane at the lower part of the coenosarc that the growth of the coral ‘skeleton’ takes place. It’s infact, an external and communal skeleton.

The coral substance is calcium carbonate in the form of calcite with some 3 per cent magnesium carbonate, organic matter and traces of iron. The S.G. is 2.68, a little lower than that of pearl and its hardness is usually given 3 1/4 on Moh’s scale, fractionally softer, than fluorite, which is 4 Moh’s Scale

The material has played an important part in jewellery for 2000 years and was particularly popular with the Romans. For them it was in a sense a local industry, since the chief source of precious coral from the earliest times to the present day has been the Mediterranean sea, where it is extensively dredged off the coasts of Aljeria, Tunis, Corsica, Sardinia, Sicily and Spain. The Japanese home waters have also provided fine quality coral especially of an ox-blood colour. But it’s the Italians who have always had almost a monopoly in the fashioning and trading in coral jewellery.

TYPES OF CORAL

Coral is found in many varieties and colours. The most ’ used in jewellery is Corallium rubrum or Corllium nobile. Generally the corals occur in colours of red, pink, white, black and vary rarely blue. Most of the coral used since antiquity as an ornamental material comes from the calcareous skeletons of colonies of marine organisms of the phylum Cnidaria order Corganacea, genus corallium.

The most famous of these organisms is Corallium rubrum, which lives in the waters of the Mediterranean and despite its name, provide not only red, but orange, pink and white coral. Similar to this are Corallium elatius, C. Japonicum and C. Secundum, which mainly live off the coasts of Japan, China, Indochina, the Phillippines, and other archipelagos of the Indian and Pacific oceans. Other corals are occasionally used for ornamental purposes as well. They are the calcareous skeleton of Helipora coerulea of the order Coenothecalia, which produces what is known as blue coral, and the horny, proteineous skelton of corals of the orders Gorganacea, Zoanthinaria and Anthipatharia, which supply so-called black coral.

Besides the white to red coral of Corallium rubrum, a white coral Oculinacea vaseuclosa, has been fashioned for ornamentation. A black coral known as ‘Akabar’ or King’s coral (Antipathes spiralis), and a blue coral known as ‘Akori’ coral, has been fished off the Cameroon coast. Both these types are horny in nature. A specimen of black coral was found to have a density of 1.34 and a vague refractive index of 1.56; this indicates that the material is organic and may be conchiolin. Unlike true precious coral this type does not effervesce with acid. This black coral appears to have first been found in 1958 off the Island of Maui of the Hawaiian group, and later, during 1966 off the coast of Queensland, Australia. Both have been fashioned and used in jewellery.

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