Opal

Opal is a <a href="/facts/Hydrate/dev82l69">hydrated</a> <a href="/facts/Amorphous/YTIzmrR0">amorphous</a> form of <a href="/facts/Silicon_dioxide/A3WILp6J">silica</a> (SiO2·nH2O); its <a href="/facts/Water_content/N0MlAYmb">water content</a> may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a <a href="/facts/Mineraloid/XUKJTtLE">mineraloid</a>, unlike <a href="/facts/Crystal/e2R2mk3E">crystalline</a> forms of silica, which are considered <a href="/facts/Mineral/LkUFbNmv">minerals</a>. It is deposited at a relatively low temperature and may occur in the <a href="/facts/Fissure/GY5JGTkv">fissures</a> of almost any kind of <a href="/facts/Rock_(geology)/FXCJkmop">rock</a>, being most commonly found with <a href="/facts/Limonite/KCPspvHH">limonite</a>, <a href="/facts/Sandstone/RP8B6fqL">sandstone</a>, <a href="/facts/Rhyolite/yKeR5Lut">rhyolite</a>, <a href="/facts/Marl/xm7XVXde">marl</a>, and <a href="/facts/Basalt/uhbwGPTb">basalt</a>.
The name opal is believed to be derived from the <a href="/facts/Sanskrit/ychfjhlx">Sanskrit</a> word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).
There are two broad classes of opal: precious and common. Precious opal displays play-of-color (<a href="/facts/Iridescence/ykc1opK8">iridescence</a>); common opal does not. Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." The internal structure of precious opal causes it to <a href="/facts/Diffraction/Mk80xXHT">diffract</a> light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered the rarest, while white, gray, and green opals are the most common.

Opal open-in-new

Opal