There is a small but important class of gemstones that are not really stones at all. Rather than being minerals with a crystalline structure, their origin is organic - they formed from biological processes, whether animal or vegetable. The class of organic gems is small but includes a number of unusual varieties that are important in the gem trade; amber, coral, jet, ivory and pearl.
Coral is a branching skeleton-like structure built by small marine animals known as coral polyps. Gem-quality coral is related to reef-forming coral, but the most valuable coral variety is found in the Mediterranean, the Red Sea and in coastal Japan. Colors vary and can be bright-red to dark-red, orange-red, orange-pink, pink, white, blue or black. Unworked coral is dull, but when polished, coral exhibits an attractive vitreous luster.
Precious coral is a species of coral that grows on rocky sea beds, typically in dark environments, either in the depths of the ocean or in dark caverns. The coral skeleton is composed of hard calcium carbonate colored by carotenoid pigments. Coral typically exhibits a range of warm reddish-pink colors ranging from salmon-pink to deep-red. The word 'coral' is also used to refer to such colors. However, gem-quality coral is also found in white, black and blue.
Due to its intense coloration and luster, precious coral has been harvested since antiquity for decorative use. Coral jewelry has been found in ancient Egyptian and prehistoric European burials. Most precious coral is harvested in the Mediterranean Sea, especially in Sardinia. Deposits are also found in the Pacific, in Japan, Taiwan and Australia.
Since coral is an organic material, it is not especially durable. Coral is reasonably hard, but not nearly as hard as many gemstones; it has a rating of only 3 to 4 on the Mohs scale. Coral is sensitive to heat, acid and high temperatures, and the color can fade slightly with wear. White and red coral has a specific gravity or density of 2.60 to 2.70 and a refractive index of 1.486 to 1.658. Coral is translucent to opaque.
Coral is polished with fine-grained sandstone and emery, and then finely polished with felt-wheels. It is used for beads, cabochons, ornamental objects and sculptures. You will sometimes see branch-like pieces that are drilled and strung on spiky necklaces.
- First Published: July-02-2008
- Last Updated: February-05-2015
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