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, marl and basalt. 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.
The water content is usually between three and ten percent, but can be as high as
twenty percent. Opal ranges from clear through white, gray, red, orange, yellow,
green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the
reds against black are the most rare, whereas white and greens are the most common.
These color variations are a function of growth size into the red and infrared
wavelengths. Opal is Australia's national gemstone.
Source: Opal on WikiPedia
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