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Boulder Opal

The Mineral opal

The rich play of color in some Opals gives them unsurpassed splendor and mystique. For this reason, Opal is one of the most fascinating and fabled of gemstones.

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.
Chemical Formula SiO2 · nH2O
Composition Hydrous silicon dioxide. The water can range from 3% to 21% of the total weight, but is usually between 6% to 10%.
Color Colorless, white, yellow, orange, red, purple, blue, green, gray, brown, and black. These are some of the base colors of Opal. Certain opals display different colors when viewed from different directions, or when the stone is turned, or when the light source is moved. This phenomenon, called play of color, gives a stone color flashes, or schillers of different colors which vary from stone to stone. Opal also occurs multicolored and banded.
Streak White
Hardness 4.5 - 6.5
Crystal System Amorphous
Crystal Forms
and Aggregates
Opal is amorphous and does not occur in any crystals, except when it forms as a pseudomorph after another mineral. Opal habits include massive, botryoidal, reniform, stalactitic, earthy, nodular, as veins, in crusts, and in accumulating mounds. It often pseudomorphs after organic matter such as wood, shell, and bone.
Transparency Transparent to opaque
Specific Gravity 1.98 - 2.25
Luster Usually vitreous, but may also be pearly, waxy, or resinous.
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Other ID Marks 1) Rich play of color in some specimens.
2) Often fluoresces, usually bright green; but also light green, light blue, purple, and white.
In Group Silicates; Tectosilicates; Silica Group
Striking Features Form, hardness, and opalescence
Environment Forms in all mineral environments, especially igneous environments.
Rock Type Igneous, Sedimentary, Metamorphic
Popularity (1-4) 1
Prevalence (1-3) 1
Demand (1-3) 1


Opal has an abundance of varieties. Some, such as Hyalite and Fire Opal, are universally accepted, while many others are known but not as standardized. There are also many names invented by dealers, and Opal unfortunately suffers from an over-abundance of prefixes. The list below attempts to delineate those names that have been encountered or documented on more then one occasion. There are many other variety names that are rarely used that are not listed here. This list tries to be as exhausting as possible without overburdening the reader.
 -  Opal from Andamooka, South Australia.
 -  Form of Opal with color bands.
 -  Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color.
 -  Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.
 -  Opaque, highly porous type of Common Opal.
 -  Orange-red to bright red variety of Mexican Fire Opal.
 -  Opal similar to Prase Opal, but with a lighter green hue.
 -  Opal similar to Prase Opal, but with a golden-green color.
 -  Transparent Opal from Mexico with an intense red, green, blue, and yellow play of color.
 -  Any Opal without play of color.
 -  Opal where the play of color is visible only when a light source is behind the stone.
 -  High quality Opal from Coober Pedy, South Australia.
 -  Transparent to translucent Opal where play of color is visible on the surface and in the interior of the stone.
 -  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.
 -  Yellow-orange to red Opal.
 -  Opal with large schillers that abruptly appear and disappear as the stone is rotated.
 -  Opal formed from deposition of hot water springs. (Also called Perlite, Fiorite, or Geyser 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.
 -  Transparent to translucent Opal with an orange to orange-brown, honey-colored background. It may or may not display play of color.
 -  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.
 -  Colorless, light yellow, or blue transparent variety of Opal, lacking play of color.
 -  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.
 -  A transparent Precious Opal with a gelatinous appearance and a bluish sheen. Jelly Opal may also refer to a colorless, transparent Common Opal.
 -  Opal with a lemon-yellow color.
 -  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.
 -  Thin layer of Opal on host rock (matrix).
 -  Opaque, grayish-brown form of Opal. Also known as Liver 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.
 -  Opal with a milky-white color.
 -  Opal containing inclusions resembling moss.
 -  Opal from the Virgin Valley (Humboldt Co.), Nevada.
 -  Opal resembling banded Onyx.
 -  Thin layer of Opal on host rock (matrix).
 -  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.
 -  Opal with very small, pinhead-size color flashes.
 -  Yellow-orange to red Opal with play of color.
 -  Any Opal with a play of color.
 -  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.
 -  Opal pseudomorph after a shell.
 -  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.
 -  Opal from the Virgin Valley (Humboldt Co.), Nevada.
 -  Synonym of Jelly Opal.
 -  Yellow to brown Opal with a waxy luster.
 -  Opal from the White Cliffs, New South Wales, Australia.
 -  Precious Opal with a light colored body color, such as white, yellow, and beige.
 -  Any Opal that formed a pseudomorph after wood from a tree, and retains the original shape and appearance of the wood.
 -  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.

Opal is one of the most precious gemstones. Black Opal is the most valuable and desired form, but White Opal and Precious Fire Opal can also be quite costly. Opals gemstones are cut and polished into cabochons, and in a few rare cases are faceted into several cuts.

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.

Common Opal (Opal without play of colors) is very common and occurs worldwide. It is beyond the scope of this guide to list all the significant Opal occurrences. Only important deposits of Precious Opal are mentioned here.

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
Queensland, Australia has numerous Opal producing areas in remote, deserted lands (sometimes hundreds of miles from the nearest community). Some of the most productive Queensland Opal fields are Bull Creek, Hungerford, Opalton, Opalville, Quilpie and Yowah.

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).


The hardness and forms of Opal distinguish it from all minerals. Some Opals resemble certain types of Chalcedony, but the hardness difference can distinguish the two.

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Much information on this page was generously contributed by Allan W. Eckert, the author of the most exhaustive and detailed book on opals, The World of Opals.

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