About
Opal, being amorphous, is not truly a mineral but a mineraloid. One of the scientifically accepted standards defining a mineral is that a mineral must have a crystal structure, which opal lacks. Despite this, virtually all scientific references, including the acclaimed Dana's System of Mineralogy, categorize Opal together with the true minerals.
Many theories attempted to explain the cause of the play of color in Opal. In the 1960's, the reason of the color play was discovered with the aid of the electron microscope, where it was determined that Opal is composed of tiny silica spheres that can be arranged in an orderly pattern. This diffracts the light entering the stone into the spectral colors. A light wave diffracted through the Opal causes a color sheen or scintillation in the stone. The density and pattern of the aligned silica spheres are responsible for the different colors refracted in the Opal. Common Opal lacks this effect, since its spheres are disordered or too compact to permit the light from refracting.
A condition called crazing affects certain Opals, causing them to form internal and external cracks. Crazing is a particularly interesting phenomenon, since it lacks consistency and is unpredictable. Although it can occur at random, it usually strikes when an Opal removed from damp conditions is allowed to dry too quickly, or when an opal is exposed to sudden intense light (or a combination of these factors). Crazing may also take place when an opal is subject to vibration, as during the cutting and polishing of a specimen. The severity of the crazing and the time it takes to "craze" varies among specimens. The origin of the specimen is often a determining factor to its resistance to crazing. A very gradual drying process over months or even years can in some cases effectively stabilize the stone and allow it to be cut and polished with a substantially reduced risk of crazing.
Uncut Opals are often stored in water; this reduces the chance of crazing. Once a specimen is taken out of the water its susceptibility increases. Opal stored in water should not be taken out of the water for more than several minutes at a time. Cutting or polishing Opals, especially Opals from localities notorious for crazing, is a risky process; it is a matter of chance if the Opals will craze or not. To further protect Opals from crazing, they should not be washed with chemicals or detergents and should not be subject to sudden changes in temperature or lighting.
For additional information, see the gemstone section on Opal.
Crystal Forms & Aggregates
Other ID Marks
2) Often fluoresces, usually bright green; but also light green, light blue, purple, and white.
Striking Features
Environment
Varieties
✓ common · ✗ uncommon
- Black Opal— Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color.
- Boulder Opal— Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.
- Common Opal— Any Opal without play of color.
- Fire Opal— Yellow-orange to red Opal.
- Hyalite— Colorless, light yellow, or blue transparent variety of Opal, lacking play of color.
- Mexican Fire Opal— Form of transparent Opal from Mexico, usually with an orange or red colors, used as a as a gemstone. Mexican Fire Opal usually refers to the form without play of color. If it exhibits a play of color, it is known as Precious Fire Opal.
- Moss Opal— Opal containing inclusions resembling moss.
- Precious Fire Opal— Yellow-orange to red Opal with play of color.
- Precious Opal— Any Opal with a play of color.
- White Opal— Precious Opal with a light colored body color, such as white, yellow, and beige.
- Wood Opal— Any Opal that formed a pseudomorph after wood from a tree, and retains the original shape and appearance of the wood.
- Andamooka Opal— Opal from Andamooka, South Australia.
- Banded Opal— Form of Opal with color bands.
- Cachalong Opal— Opaque, highly porous type of Common Opal.
- Cherry Opal— Orange-red to bright red variety of Mexican Fire Opal.
- Chloropal— Opal similar to Prase Opal, but with a lighter green hue.
- Chrysopal— Opal similar to Prase Opal, but with a golden-green color.
- Claro Opal— Transparent Opal from Mexico with an intense red, green, blue, and yellow play of color.
- Contra Luz Opal— Opal where the play of color is visible only when a light source is behind the stone.
- Coober Pedy Opal— High quality Opal from Coober Pedy, South Australia.
- Crystal Opal— Transparent to translucent Opal where play of color is visible on the surface and in the interior of the stone.
- Diatomite— Opal replacement of microscopic shells of diatoms (a type of microscopic organism) that are clustered together in a rock-like formation. It is white, opaque, and chalky in texture. Synonym of Tripolite, Fuller's Earth, and Diatomaceous Opal.
- Flash Opal— Opal with large schillers that abruptly appear and disappear as the stone is rotated.
- Geyserite— Opal formed from deposition of hot water springs. (Also called Perlite, Fiorite, or Geyser Opal.)
- Harlequin Opal— Opal in which the play of color is arranged in a consistent harlequin, diamond-shaped, or rectangular-shaped pattern that is very vivid. Harlequin Opal is one of the rarest and most prized forms of Opal.
- Honey Opal— Transparent to translucent Opal with an orange to orange-brown, honey-colored background. It may or may not display play of color.
- Hungarian Opal— Describes Opal from the old sources in Hungary (as well as other places in Europe such as the Czech Republic). This term has become corrupted and is sometimes used to describe White Opal from other locations as well.
- Hydrophane Opal— White, opaque, highly porous form of Opal, that when placed in water allows the water to seep into it. This causes the stone to become transparent and almost invisible while in the water.
- Jelly Opal— A transparent Precious Opal with a gelatinous appearance and a bluish sheen. Jelly Opal may also refer to a colorless, transparent Common Opal.
- Lemon Opal— Opal with a lemon-yellow color.
- Lightning Ridge Opal— Opal from Lightning Ridge (New South Wales), Australia. Although different forms of Opal are found at Lightning Ridge, this term often represents the high quality Black Opal found there.
- Matrix Opal— Thin layer of Opal on host rock (matrix).
- Menilite— Opaque, grayish-brown form of Opal. Also known as Liver Opal.
- Milk Opal— Opal with a milky-white color.
- Nevada Opal— Opal from the Virgin Valley (Humboldt Co.), Nevada.
- Onyx Opal— Opal resembling banded Onyx.
- Opal Matrix— Thin layer of Opal on host rock (matrix).
- Pineapple Opal— Opal pseudomorph after Ikaite that resembles a pineapple. It is found only in White Cliffs (New South Wales), Australia. The pseudomorphed mineral was originally thought to be Glauberite, but studies now prove it to be the rare and unstable mineral Ikaite.
- Pinfire Opal— Opal with very small, pinhead-size color flashes.
- Seam Opal— Opal found in the seams or large cracks of rock. (May also specifically refer to masses of white Common Opal containing bands of precious White Opal.
- Shell Opal— Opal pseudomorph after a shell.
- Tabasheer— Opal occurring as an organic byproduct. It forms by the hardening of a secretion issued from certain bamboo, forming a porous, rounded mass of Opal.
- Virgin Valley Opal— Opal from the Virgin Valley (Humboldt Co.), Nevada.
- Water Opal— Synonym of Jelly Opal.
- Wax Opal— Yellow to brown Opal with a waxy luster.
- White Cliffs Opal— Opal from the White Cliffs, New South Wales, Australia.
- Yowah Nut— Small, rounded form of Opal from Yowah (Queensland), Australia in a nodule embedded in ironstone. Closely related to Boulder Opal, it occurs most often as walnut-sized ironstone nodules containing pockets, veins, or sprinklings of vivid Precious Opal.
Uses
Opal is also extremely popular among mineral collectors and museums compete to get the finest specimens. Common Opal has no industrial or commercial use, except for those specimens that are brightly fluorescent and are collected by fluorescent mineral collectors.
Noteworthy Localities
Most Precious Opal is mined in Australia, the U.S., and Mexico. Some of the most famous Opal deposits are in Australia, and below are the most significant Australian localities:
- Andamooka, South Australia
- Coober Pedy, South Australia
- Lightning Ridge, New South Wales
- Mintabie, South Australia
- White Cliffs, New South Wales
In Mexico, Precious Opals and Fire Opals come from several deposits. The most important are near Queretaro, in Queretaro state, and near Magdalena, in Jalisco state.
The U.S. has some of the most outstanding Opal occurrences. Virgin Valley, Humboldt Co., Nevada is rich in Opal mines producing all types of Precious Opal. Also worthy of mention are the Spencer area Opal mines in Clark Co., Idaho; Opal Butte, Morrow Co., Oregon; and the Last Chance Opal Mine, Kern Co., California. In Canada, a notable deposit exists in Vernon, British Columbia.
Other significant worldwide Precious Opal deposits are in Ethiopia (Menze Gishe), the Czech Republic, Slovakia (Dubnik), Hungary, Brazil (Piaui), and Honduras (Gracious O Dios).