Although opal is regarded as a valid mineral species for historical reasons, it is not a true mineral in the accepted sense of the word. It is composed of amorphous silica. A gemstone, opal has numerous varieties such as fire opal which has intense orange-to-red reflections, and wood opal which occurs in petrified wood. It may be deposited by hot springs at shallow depths, but the largest accumulations are as siliceous tests of silica-secreting organisms.

Mineral Photos courtesy of R.Weller/Cochise College