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Opal is a mineraloid gel, the water content sometimes is as high as 20%, but is usually between three and ten percent. Opal color ranges from clear through white, gray, red, yellow, green, blue, magenta, brown, and black. Of these hues, red and black are the most rare and dear, whereas white and green are the most common. The word opal comes from the Latin OPALUS, by Greek OPALLIOS, and is from the same root as Sanskrit UPALA(s) for "stone", originally a millstone with UPARA(s) for slab.
Opal is truly amorphous, but precious opal has a structural element. At the micro scale precious opal is composed of silica spheres, that produce a variable interplay of internal colors by causing the interference and diffraction of light passing through the microstructure of opal. The term opalescence is commonly and erroneously used to describe this unique phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common opal, that does not show a play of color.
Besides the gemstone varieties that show a play of color, there are other kinds of common opal that don't have this effect, such as the milk opal, milky bluish to greenish; resin opal, honey-yellow with a resinous lustre; wood opal; menilite brown or gray; hyalite, a colorless glass-clear opal; geyserite, deposited around hot springs or geysers; and diatomite, the accumulations of diatom shells or tests. Other varieties of opal are: fire opal is a translucent to semi-opaque stone that is generally yellow to bright orange and sometimes nearly red; peruvian opal is a semi-opaque to opaque blue-green stone found in Peru; boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix.
Until the nineteenth century the only source of precious opal known to Europeans was the mining district in Slovakia. Opal without play of color is very common and can be found all over the world, unlike precious opal deposits that are in greater scope found today only in Australia, USA, and Mexico. Australia produces around 97% of the world's opal. Boulder opal is found sporadically in western Queensland, Australia. Fire opal is found mostly in Mexico and Mesoamerica. Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.
1. The opal is the official gemstone of South Australia and the Commonwealth of Australia.
2. Australia country's women's national team in basketball is nicknamed "The Opals".
3. The official state gemstone for Nevada is precious black Fire Opal, in recognition of the black opal found in Virgin Valley, Humboldt County, Nevada.
4. The largest black opal in the Smithsonian Museum, possibly worth in excess of $1 million, comes from the Royal Peacock Opal Mine in the Virgin Valley.