Opal is a hydrated amorphous form of silica (SiO2·nH2O). 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. Opal is the national gemstone of Australia.
There are two broad classes of opal: Precious and Common
Precious opal display play-of-color (iridescence), common opal does not. Play-of-color is defined as “a pseudochromatic 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 to be the rarest, whereas white, gray and green are the most common. Besides the gemstone varieties that show a play of color, the other kinds of common opal include the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality), resin opal, which is honey-yellow with a resinous luster, wood opal, which is caused by the replacement of the organic material in wood with opal menilite, which is brown or grey, hyalite, a colorless glass-clear opal sometimes called Muller’s glass, geyserite, also called siliceous sinter, deposited around hot springs or geysers, and diatomaceous earth, the accumulations of diatom shells or tests. Common opal often displays a hazy-milky-turbid sheen from within the stone. In geology, this optical effect is strictly defined as opalescence which is a form of adularescence.