opal, a mineral consisting of poorly crystalline to amorphous silica, SiO2·nH2O; the water content is quite variable but usually ranges from 3% to 10%. Common opal is usually colorless or white, but it may be gray, brown, yellow, or red; the color is due to fine-grained impurities. Opal is formed at low temperatures from silica-bearing waters and can occur in fissures and cavities of any rock type. Precious, or gem, opal has a rich iridescence and remarkable play of changing colors, usually in red, green, and blue. This is the result of a specific internal structure consisting of regularly packed uniform spheres of amorphous silica a few tenths of a micron in diameter; sphere diameter and refractive index determine the range of colors displayed. The greater part of the world's supply of precious opal comes from the Coober Pedy and Andamooka fields in South Australia. The original source, known in Roman times, was in what is now E Slovakia. Precious opal has also been mined in Honduras, Mexico, and the Virgin Valley in Nevada. Fire opal is a bright red transparent or translucent opal that may or may not show a play of color.
Precious opal
Precious opal shows a variable interplay of internal colours and does have an internal structure. At the micro scale precious opal is composed of hexagonal or cubic closely packed silica spheres some 150 to 300 nm in diameter. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of opal (Klein and Hurlbut, 1985, p. 444). In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.
The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a cap of clear quartz (rock crystal) on top, which takes a high polish, and acts as a protective layer for the comparatively delicate opal.
Opal is symbolic of love, death, and breaking up. In the Medieval Times, the opal stone was given by a male to an unwanted female mate.
Common opal
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); 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; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
