Mineralogically, opal is a hydrous silicon dioxide with a chemical composition of SiO2.nH2O. It is amorphous, without a crystalline structure, and without a definite chemical composition (it contains a variable amount of water). Therefore opal is a "mineraloid" rather than a "mineral." Opal is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The showers soaked deep into the ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward. During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
Opal is a very common material, found throughout the world. Most opal is "common opal" or opal that lacks the colorful flashes known as "play-of-color". The rare specimens of opal that exhibit a play-of-color are known as "precious opal". If the play-of-color is of high quality and large enough to cut, the material can be used to produce valuable gemstones.
Play-of-color occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of ping-pong balls in a box. As the lightwaves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colors of the rainbow, called spectral colors. Play-of-color is the result.
If a precious opal is examined under bright light, play-of-color can be observed in three situations: 1) when the stone is moved, 2) when the light source is moved, or, 3) when the angle of observation is changed. A nice precious opal can flash every color of the spectrum with an intensity and quality of color that surpasses the fire of diamond. The best opals command prices per carat that rival expensive diamonds, rubies, sapphires, and emeralds.
The word "opalescence" is often misused and it differs from the "play-of-color". The common definition given for opalescence is "the pearly luster of common opal". In truth, most common opal does not have a pearly luster, even when it is polished.
Although experts divide gem opals into many different categories, five of the main types are:
White or light opal: Translucent to semitranslucent, with play-of-color against a white or light gray background color, called bodycolor.
Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
Fire opal: Transparent to translucent, with brown, yellow, orange, or red bodycolor. This material—which often doesn’t show play-of-color—is also known as “Mexican opal.”
Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem
Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.
Opal was rare and very valuable in antiquity. Because opal has the colors of other gems, the Romans thought it was the most precious and powerful of all. The Bedouins believed that opals contained lightning and fell from the sky during thunderstorms. In Europe, it was a gem prized by royalty. Until the opening of vast deposits in Australia in the 19th century the only known source was Červenica beyond the Roman frontier in Slovakia.
Significant deposits of opals around the world can be found in the Czech Republic, Canada, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Ethiopia and Nicaragua, however Australia produces 95% of the world's precious opal and it is its official national gemstone. In US, opals are mainly found in the states of Nevada, Oregon and Idaho. In Virgin Valley in Nevada there are black fire opals.
Opal is October birthstone.