The variety of coral used in jewelry is known as corallium rubrum, or corallium nobile. The masses grow in much the same shape as a shrub but smaller, the trunk never over 2 inches across. They grow in indefinite directions, and taper towards a slightly rounded end.
Opaque and fairly soft (the hardness is only about 3 1/4), coral may be filed and cut fairly easily. The specific gravity is about 2.65 and thus approximates to pearl. Colors vary and they are not always homogeneous, but reddish shades and white are by far the most usual. The salmon pink, sometimes marred by black streaks, and the deep red are the commonest shades seen in jewelry, the rose pink and the ox-blood red being rarer. The rose-pink is the most expensive, and a fine quality has a very attractive pale pink hue which is not universally appreciated. Yellow and black coral are not used, such colors being of a dirty complexion and not definite or bright. Similarly, cracked or pitted pieces are to be found only in the cheapest articles.
The white coral is sometimes stained, and the salmon pink is deepened to a red, particularly by the Japanese. But such treated material does not retain its color, which soon reverts to its original shade. Certain colors are characteristic of certain seas, and experts have no difficulty in ascertaining their origin, even in the finished article. The cause of the various colors is not definitely known, but it is thought that calcium and magnesium carbonates with a small amount of iron oxide may be the agents.
The coastal waters of the Mediterranean Sea are fished regularly for coral, for here the polyp abounds. The Italians hold the monopoly in this industry, and material is drawn from the coasts which border Algeria, Tunis, Corsica, Sardinia, Sicily, and Spain. A. small quantity comes from the Adriatic Sea, and an increasing iimount from the seas around Japan.
Just before 1939, it was said that nearly 80 per cent of the world’s supply of coral originated from Japanese waters, but this figure would be hard to substantiate. The headquarters was Kachinoshima, some 160 miles due south from Nagasaki. Some secret was made of this town and its industry, but the Italians seemed to buy most of the coral and work it in Naples. Japan’s export is now restricted, however, and prices are regulated. Postwar production figures are not available, but it is unlikely that more than a small quantity has been marketed as yet.