Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as 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. necklace gold with diamond
Opal is the national gemstone of Australia. The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed, it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the black opals are the rarest, whereas white and greens are the most common. Opals vary in optical density from opaque to semitransparent.
Common opal does gold chain and diamond pendant not show the display of colors that precious opal does, but it may show a “play of color” that is similar to that exhibited by certain oils on water. It is frequently used as a gemstone in jewelry, and has also been used for cameo carving and as an inlay in woodworking.
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