Opal, a mineral, some varieties of which are valued as gems. Opal is composed of noncrystalline silica and a variable amount of water. Opal does not have a definite chemical composition or uniform atomic structure. It is not as hard as quartz, and never forms crystals. The mineral is found in deposits from hot springs and in sedimentary and volcanic rocks. Opal may be colorless, white, black, brown, gray, green, or red orange. It is translucent and has a vitreous luster.

There are two types of opal. Common opal, which is the more abundant, is of no significant commercial value. Precious opal is the October birthstone, and represents hope. It is among the most attractive and costly of minerals and gems, and is found in several colors. The black opal is the most valuable. Precious opal owes its beauty and popularity as a gem to opalescence, the rich display of colors from within the stone. Opalescence occurs because light is diffracted by tiny spaces between particles of the silica. The diffraction creates a prism-like effect, causing the light to break up into its component colors. The finest gem opals are mined in Australia, Mexico, and the western United States.

General chemical formula: SiO2H2O. Specific gravity: 2.0-2.2. Hardness: 5-6.