Opal
A species of the quartz family of minerals, from which it differs in containing 5 to 13 per cent of water. It is a precious stone, consisting principally of silica with some alumina. When first dug from the earth it is soft, but it hardens and diminishes in bulk by exposure to the air. The specific gravity varies from 1.9 to 2.5. Subspecies of this gem are called the semi and the wood opal. It is translucent, usually blue or yellowish white, and exhibits a beautiful play of brilliant colors owing to minute fissures which refract the light. Found in Hungary, Queenstown, and the United States. See MINERALS; PRECIOUS STONES.