Noncrystalline hydrous siliCOn dioxide. Opal has the same chemical composition as quartz, but contains from 1 to 2 percent water, and is not crystallized. it has a certain type of regular structure, but noi at the atomic level. being composed of alignments of tiny spheres (from 40 to 4000 it in diameter) which form a compact, three-dimensional network. The name opal is apparently derived, through the Greek opéllios, from the Sanskrit upaia, meaning "precious stone.“

Crystal system Noncrystalline.
Appearance It occurs as narrow veins of up to 10 centimeters or more, or as nodules, inside cavities or cracks in silica-rich rocks. It may also be found pseudomorphous alter other minerals. It may have a whitish to light gray. pale green, sky-blue, smoke gray, black, yellowish to orange or reddish background color. It can be semiopaque, with a vaguely porcelainlike appearance. or similar to broken glass, with shiny, conchoidal fracture. It is more often translucent and milky, with an appearance that is so characteristic, it is described as opalescent. Opals can even befully or largely transparent: such stones are usually orange-yellow to red in color. The most highly prized varieties display internal iridescence due to light diffraction by the network of tiny spheres of which they are composed. These types are collectiveiy known as noble opal or precious opal. The range of colors apparently depends on the size of the spheres, or rather, the distance between the
rows.

In gem quality precious opal, three sets of distinctions are made. The first, according to the ground color of the material, distinguishes light or white opal from dark or black opal? The second, applied to each of these two varieties. is based on the range of colors in the iridescent patches; and the third is based on the size, shape, and distribution of the patches. The transparent or semitransparent, noniridescent variety (known as common opal) is also used as a gem if it is attractively colored. Because of its orange-yellow to reddish orange color, it is known as fire opal.

Physical properties Opal has a hardness of 5.5 to 6.5 and very low density; from about 1.98 to 2.20 g/cma, depending on the water content. The refractive index-varies from 1.44 to 1.46 or a little more. It is singly refractive. it may be somewhat porous, in which case it is dangerous to immerse it in liquids other than water.

Genesis Opal is normally found in assocation with effusive magmatic rocks, such as rhyolites, andesites, and trachytes, having been deposited in cracks and cavities by aqueous fluids at low temperature. In Australia, it is found both in connection with trachytes and basalts. and in siliceous sandstone where hydrous silica has been precipitated, perhaps through alteration of feldspars by percolating waters in an environment subiected to very long periods of staple water conditions.

Occurrence One area of eastern Czechoslovakia formerly belonging to Hungary has been mining opal since Roman times and was the only source of noble opal for Europeans until the nineteenth century. Nowadays, most opal comes from Australia, where the finest quality opals are found. Other sources are Mexico and, to a lesser extent, Guatemala and Honduras. Low—value or subgem quality varieties of opal are found in many other places, especially the United States (xyloid opal or petrified wood) and iceland {where it is deposited by hot springs).