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Opal Fact Sheet

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Opals are noncrystalline gem mineral composed of hydrous silicia, and contain a percentage of water.

Opal has a hardness of between 5.5 and 6.5 and a sp.gr. of 1.9 to 2.3.

The fracture of the mineral is conchoidal and its luster varies from glassy to dull. In color the opal also shows extreme variations from white to black, and in transparency from transparent to opaque.

One of the chief characteristics of the opal is the brilliant play of colors which may be seen in superior stones. These colors result from the cracking of the original stone as it hardens and the deposition of additional opal in the cracks. The indices of refraction of the original stone and the additional deposits are frequently different and result in light interference causing a play of colors. Opal has been used as a gemstone for many centuries in spite of a superstition that the gem brings back luck to its owners.

A large number of different types of opal are known, but usually only the transparent or translucent varieties are used as gems. Gem opals include white opals; black opals; fire opals, which are yellow to red in color; girasol, which has a bluish-white opalescence, harlequin opals, which show uniform patches of contrasting colors; and lechosa opals, which have a deep-green play of color within the stone. Other types of opal include moss opal, which has inclusions of foreign material resembling moss; hydrophane, a porous, white opal which is cloudy when dry and transparent when the pores are filled with water; and hyalite, a glasslike, transparent form of opal.

Hydrous silica flows underground with water, and is deposited in cracks and cavities. It sometimes flows into buried wood or bones to form opalized fossils. In all opals, whether the background is white, black or blue, the play of other colors results from the internal refraction of light on tiny imperfections and impurities.

Most of the world's opal production is mined in Australia.

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