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Chemical Formula | SiO2 · nH2O | |
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Composition | Hydrous silicon dioxide. The water can range from 3% to 21% of the total weight, but is usually between 6% to 10%. | |
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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. The play of color in many Opals is truly exceptional and unsurpassed. Opals exhibiting such play of color are termed precious and are very valuable; those without it are termed common and have little or no value. |
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Streak | White | |
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Hardness | Opal is listed in most references with a hardness of 5½. This is not fully correct, since some Opals are as low as 4½ and some as high as 6½. | |
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Crystal Forms and Aggregates |
(Amorphous) Opal, being amorphous, is not
really 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. Many scientific
groups and references, including the acclaimed Dana's
System of Mineralogy, categorize Opal together with the
"true" minerals. For this reason, Opal -- as
well as other amorphous types that fall under the
definition of a mineraloid -- is mentioned in the mineral
section of this guide. An organic form of Opal, known as Tabasheer or Pearl Opal, is formed in some species of bamboo. Opal occurs massive, botryoidal, reniform, stalactitic, earthy, nodular, as veins, in crusts, and in accumulating mounds. It often pseudomorphs after other minerals and organic matter, such as wood, shell, and bone. |
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Transparency | Transparent to opaque | |
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Specific Gravity | Common Opal - 1.98 - 2.25 Precious Opal - 2.1 - 2.2 |
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Luster | Usually vitreous, but may also be pearly, waxy, or resinous | |
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Cleavage | None | |
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Fracture | Conchoidal | |
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Tenacity | Brittle | |
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Other ID Marks | 1) Rich play of color in some specimens 2) Common Opal sometimes fluoresces, usually bright green, but also light green, light blue, purple, and white |
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Varieties | There are many Opal varieties, each with its
own name. Some are not scientifically recognized, but are
universally used. The most important names are listed
below; a much larger list can be found in the Opal
variety page. (Opal variety page) Common Opal - Any Opal without play of color Precious Opal - Any Opal with play of color Black Opal - Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. White Opal - Precious Opal with a light colored body color (white, yellow, cream, etc.) Fire Opal - Yellow-orange to red Opal Precious Fire Opal - Yellow-orange to red Opal with play of color Hyalite - Colorless, sky-blue, or misty-blue transparent variety of Opal. |
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In Group | Silicates ; Tectosilicates ; Silica group May be classified as an oxide by a few abstract references (Oxide ; Hydroxide) |
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All About | 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. The following is a brief
explanation: Opals are composed of tiny silica spheres that when arranged in an orderly
pattern diffract 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, for its spheres are
disordered or too compact to permit the light from
refracting. The rich play of color in Precious Opal gives it unsurpassed splendor. For this reason, it is one of the most fascinating and fabled of gems. Specimens and jewels with a rich play of color command lofty prices. A condition called crazing affects certain opals, causing them to form internal and external cracks. Crazing is a particularly interesting phenomenon, for 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 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. |
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Uses | Opal is one of the most precious gemstones. Black
Opal is the most valuable and desired form, but White
Opal and Precious Fire Opal are also quite
costly. Opals are cut and polished into cabochons, and in a few rare cases are facetted into
several cuts. Opal is also extremely popular among
mineral collectors and museums compete to get the finest
specimens. Common Opal (Opal without play of
color) has no industrial or commercial use, except for
those specimens that are brightly fluorescent and are collected by fluorescent
mineral collectors. Also see the gemstone section on Opal |
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Striking Features | Form, hardness, and opalescence | |
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Popularity (1-4) | 1 | |
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Prevalence (1-3) | 1 | |
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Demand (1-3) | 1 | |
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Distinguishing Similar Minerals |
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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Commonly Occurs With |
Chalcedony | |
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Noteworthy Localities |
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 Since Queensland, Australia has numerous Opal producing areas in remote, deserted lands (sometimes hundreds of miles from the nearest community), only the names of the Opal fields are mentioned, instead of a town or village. Some of the most productive 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, the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, and Nicaragua. |
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