Coral

Specific Gravity: &nbsp;2.68
Refractive Index: &nbsp;1.49-1.66
Birefringence (DR): &nbsp;Not applicable
Luster: &nbsp;Dull to vitreous
Crystal Structure: &nbsp;Trigonal
Composition: &nbsp;Calcium carbonate or conchiolin
Hardness: &nbsp;3

Coral is made up of the skeletal remains of marine animals called coral polyps. These tiny creatures live in colonies that form branching structures as they grow, eventually forming coral reefs and atolls. The surface of these coral branches has a distinctive patterning made by the original skeleton. These are either striped or like wood grain.

Most corals – red, pink, white, and blue varieties – are made of calcium carbonate; black and golden corals are made of a horn-like substance called conchiolin. Red coral is the most valuable and has been used in jewelry for thousands of years. Dull at first, all coral has a vitreous luster when polished, but is sensitive to heat and acids and may fade with wear.

Coral is often cut en cabochon and carved to be used as pendants, earrings, and rings. It is essential to keep it away from fragrances as well to keep it safe.

Some blue corals show a distinctive pitted surface. Heliopora cacrules is a coral found in seas around the Philippines.

Coral may be imitated by porcelain, stained bone, plastic, glass, or rubber, and gypsum mixtures. Imitation slabs may be carved, polished, or fashioned as beads. These are made of vegetable ivory and stained in different colors. Imitations are generally even in color and fail to show the distinctive ‘wood grain’ structure of natural coral. Gilson of France is the pre-eminent producer of imitation coral.

It has been associated with the protection of children, and parents may still give a gift of coral to their young children.

Table of Contents

Occurrence

The most precious coral occurs in warm waters. Japanese coral is red, pink, or white. Red and pink coral is also found in the Mediterranean and African coasts, the Red Sea, and the waters off Japan and Malaysia. Black and golden coral is found off the coasts of the West Indies, the Pacific Islands, and Australia.