Opal

The mineral opal is amorphous hydrated silica, the water content sometimes being as high as 20%. Being amorphous it has no crystal form, occurring in irregular veins and masses. It has conchoidal fracture, hardness 5.5-6.6, specific gravity 2.1-2.3, and a highly variable color. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black. Often many of these colors can be seen at once, caused by interference from minute cracks in the opal which are filled with secondary silica and by thin lamellae formed inside the opal during solidification. The term "opalescent" refers to this unusual and beautiful type of color.

Besides the gemstone varieties that show a play of colors, there are other kinds of common opal such as the milk opal (a milky bluish to greenish), resin opal (honey-yellow with a resinous lustre), wood opal (caused by the replacement of the organic material in wood with opal), Menilite (brown or grey), and hyalite[?], a colorless glass-clear opal sometimes called Muller's Glass.

Opal is a mineral gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock. The word opal comes from the Latin opallus.

Opal is one of the minerals that can form fossils; the resulting fossils, though not of any extra scientific interest, appeal to collectors.

A large fraction of the world's opal comes from Australia. The town of Coober Pedy, in particular, is a major source.

See also: List of minerals