The rich play of color in some Opals gives them unsurpassed splendor and mystique. This effect gives Opal the title of the Queen of All Gems, as it can capture a rainbow in one stone. Opal is the birthstone for October. This gemstone is the national gemstone of Australia.
Opal – a mineraloid
Since Opal is amorphous, it is not technically a mineral but is classified as 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 categorize Opal together with the true minerals.
Opal basically consists of hydrated silicon dioxide (aka silica). Its water content may range from 3 to 21% by weight but is usually between 6 and 10%. There are two broad classes of Opal: precious and common. Precious Opal displays play-of-color (iridescence); Common Opal does not. The electron microscope during the mid-1960’s became the discovered reason for the color play. Precious Opal is composed of tiny silica spheres that are 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. Gives Opal the title as the Queen of Gems.
Common Opal lacks this effect, since its spheres are disordered or too compact to permit the light from refracting. Sizes of the spheres vary the color seen in precious opal. Spheres that are approximately 0.1 micron (one ten-millionth of a meter) in diameter produce violet. Spheres about 0.2 microns in size produce red. Sizes in between produce the remaining rainbow colors.
The colors and luster of Opal
The base colors of Opal (before play-of-color) are colorless, white, yellow, orange, red, purple, blue, green, gray, brown, and black. Opal often fluoresces, usually bright green, but also light green, light blue, purple, and white. Opal habits include massive, botryoidal, reniform, stalactitic, earthy, nodular, as veins, in crusts, and in accumulating mounds. It often pseudo morphs after organic matter such as wood, shell, and bone. Luster is usually vitreous, but may also be pearly, waxy, or resinous. Hardness is 4.5 – 6.5 on the Moh’s Scale.
Numerous Varieties of Opal, the Queen of all Gems
Varieties: Opal has an abundance of varieties. The list below attempts to define those names:
Andamooka Opal – Opal from Andamooka, South Australia.
Banded Opal – Form of Opal with color bands.
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.
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.
Common Opal – Any Opal without 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 clustered together in a rock-like formation. It is white, opaque, and chalky in texture. Synonym of Tripolite, Fuller’s Earth, and Diatomaceous Opal.
Fire Opal – Yellow-orange to red Opal.
Flash Opal: Infamous opal that has large schillers abruptly appearing and disappearing when rotated.
Geyserite – Opal formed from deposition of hot water springs. (Also called Perlite, Fiorite, or Geyser Opal.)
Harlequin Opal: A variety with a diamond-shaped, harlequin, or rectangular-shaped vivid pattern playing with color. 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). When used to describe white opal from other localities, the term becomes corrupted.
Hyalite – Colorless, light yellow, or blue transparent variety of Opal, lacking play of color.
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.
Mexican Fire Opal – Form of transparent Opal from Mexico, usually with an orange or red colors, used 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.
Milk Opal – Opal with a milky-white color.
Moss Opal – Opal containing inclusions resembling moss.
Nevada Opal – Opal from the Virgin Valley (Humboldt County), 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.
Precious Fire Opal – Yellow-orange to red Opal with play of color.
Precious Opal – Any Opal with a play of color.
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 County), 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.
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.
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.
Where Opal, the Queen of Gems is Found
Noteworthy Localities of Precious Opal: The primary sources of precious opal today are in Australia and Ethiopia. The most significant Australian localities are Andamooka, South Australia; Coober Pedy, South Australia; Lightning Ridge, New South Wales; Mintabie, South Australia; and 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.
Opal from Ethiopia first appeared in 1994, with the discovery of precious opal in the Menz Gishe District, North Shewa Province. This Shewa Province opal was mostly dark brown in color and tended to crack which made it unpopular in the gem trade. In 2008, a new opal deposit was found near the town of Wegel Tena, in Ethiopia’s Wollo Province. Wollo opal has become the dominant Ethiopian opal in the gem trade.
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 United States has several outstanding Opal occurrences. Virgin Valley, Humboldt County, Nevada is rich in Opal mines producing all types of Precious Opal. Also worthy of mention are the Spencer area Opal mines in Clark County, Idaho; Opal Butte, Morrow County, Oregon; and the Last Chance Opal Mine, Kern County, California. In Canada, a notable deposit exists in Vernon, British Columbia.
Other significant worldwide Precious Opal deposits are in the Czech Republic, Slovakia (Dubnik), Hungary, Brazil (Piaui), and Honduras (Gracious O Dios).
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Article by Bill Jones, Sidewinder Minerals