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.

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. Precious opal displays play-of-color ( iridescence) common opal does not. There are two broad classes of opal: precious and common. The name opal is believed to be derived from the Sanskrit word upala ( а¤‰а¤Єа¤І), which means 'jewel', and later the Greek derivative opГЎllios ( бЅЂПЂО¬О»О»О№ОїП‚), which means 'to see a change in color'. 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. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. nH 2O) its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.Opal is a hydrated amorphous form of silica (SiO 2 Hot salt water, bases, methanol, humic acid, hydrofluoric acid

Mexican opal may read as low as 1.37, but typically reads 1.42вЂ“1.43 Д«lack or white body color: inert to white to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce Single refractive, often anomalous double refractive due to strain A rich seam of iridescent opal encased in matrixД¬olorless, white, yellow, red, orange, green, brown, black, blue, pink

Black fire opal