Image 15A rich seam of iridescent opal encased in matrix
Opal is a
hydrated
amorphous form of
silica (SiO
2·
nH
2O); its
water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a
mineraloid, unlike
crystalline forms of silica, which are considered
minerals. It 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 name
opal is believed to be derived from the
Sanskrit 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 (
iridescence); 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
diffract 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. (
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