Opal is a mineraloid gel which is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt.
The water content is usually between three and ten percent, but can be as high as 20%. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare and dear, whereas white and greens are the most common; these are a function of growth size into the red and infrared wavelengths—see precious opal. Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upálá[s] for "stone", originally a millstone with upárá[s] for slab. (see Upal). Opals are also Australia's national gemstone.
Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors.
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