Coral
Coral ranks far below pearl and meets with but limited appreciation. It is common enough in warm seas, but the only kind which finds its way into jewellery is the rose or red-coloured coral—the noble coral, Corallium nobile or rubrum. It consists of the axial skeleton of the coral polyp, and is built up of hollow tubes fitting one within the other. The composition is mainly calcium carbonate with a little magnesium carbonate and a small amount of organic matter. The former of the mineral substances is in the form of calcite, and the crystals are arranged in fibrous form radiating at right angles to the axis of the coral. The specific gravity varies from 2·6 to 2·7, being slightly under that of calcite, and the hardness is somewhat greater, being about 3¾ on Mohs’s scale.
The best red coral is found in the Mediterranean Sea off Algiers and Tunis in Africa, and Sicily and the Calabrian Coast of Italy. The industry of shaping and fashioning the coral is carried on almost entirely in Italy. Coral is usually cut into beads, either round or egg-shaped, and used for necklaces, rosaries, and bracelets. The best quality fetches from 20s. to 30s. per carat.