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Roger Weller, geology instructor                     

Michael Schweska
Physical Geology
Spring 2005

                                       October Birthstone-Opal

The gemstone Opal is the official October birthstone as adopted by the American National Association of Jewelers in 1912. It is also the birth stone for the Zodiac sign of Libra. Opal is suggested as a gem to give on the 13th wedding anniversary. Opal is 5.5 - 6 on the Mohs scale of hardness. Most opal is 50-65 million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. Opal has been mined for centuries, at least since Roman times when they extracted the Opal from areas now within the Czech Republic. The Aztecs made use of local Mexican sources as did the Spaniards when they exported the material back to Europe. Today most precious Opal comes from Australia with significant sources from Mexico and the Western United States. Australia is famous for its white and black precious Opal. Black Opal comes from the Lightning Ridge region of Northern New South Wales. Opal It is the most rare and most valuable opal. Fine quality black Opals often cost more per carat than Diamonds. The term Black Opal is misleading because the Opal is not actually black, but instead has a very dark base. This dark base enhances the brilliant colors know as "fire". Fire Opal, the bright red, reddish-yellow, orange body colored Opal is the second most important Opal commercially. Until recently, the best Fire Opal came from Mexico. Boulder Opal occurs in the boulder opal belt, an area between the New South Wales border and northwest Queensland. Boulder opal forms on a dark ironstone base (the host rock) and occurs as a thin uneven layer adhering to the ironstone. Because of the uneven layers, sometimes part of the ironstone is visible on the surface of the stone. Queensland Boulder Opal accounts for a small percent of the world market, and because of its brilliance and rarity can bring up to $1000 a carat. Click here for a picture in natural form. Click here or here for more polished and natural specimens.


Opal has over one hundred variety and trade names, but the list of accepted or commonly used names is much shorter. The most important and most widely known Opal is the precious Opal. Precious Opal may be subdivided further by color modifiers, white, black, pinks, and blue, which describe the body color of the Opal. When the Opal is mixed through the ironstone it is called matrix Opal. Unlike other Opals which can be brittle, heat sensitive, break and scratch easily (some varieties self-destruct through the loss of water), Boulder Opal is very durable due to the ironstone backing the Opal forms on. Because water content within the Opal is very low it almost never cracks or crazes as it ages. Opal's name evolved from the Roman word "opalus" from the Greek word "opallios" meaning "to see a change of color". The Greek word was a modification of the ancient Indian Sanskrit name for Opal, "upala", which meant "precious stone". If one spoke in mixed tongues, then Opal would be "opallios upala", "to see a change of color precious stone".


Opal's chemical formula is SiO2 - nH2O, Hydrated Silica. Opal forms as silica from decomposing rocks mixed with ground water which creates a silica gel that collects and hardens in underground cavities and fissures. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 - 10% and greater. This water can help geologists determine the temperature of the host rock at the time the Opal formed. Although there is no crystal structure, (meaning a regular arrangement of atoms) Opal does possess a structure. Random chains of silicon and oxygen are packed into extraordinarily tiny spheres. These spheres in most Opals are irregular in size and inconsistent in concentration. Yet in Precious Opal, the variety used most often in jewelry, there are many organized pockets of the spheres. These pockets contain spheres of approximately equal size and have a regular concentration, or structure, of the spheres. This has the effect of diffracting light at various wavelengths, creating colors. Each pocket produces a different color, with a different intensity depending on the angle from which a viewer sees it. There are two distinct types of Opal, common and precious. The way the silica particles form determines which type. In precious Opal, the silica particles are packed in regular rows and layers. Moving the stone causes light to diffract, or split, as it grazes the Opal surface. This light diffusion shows iridescent flashes of green, blue, aqua and sometimes yellowish or red colors which are referred to as "fire".

Boulder Opals are graded into specific groups which have been defined by the Australian Gemological Association:
Boulder black opal
Boulder crystal opal
Boulder light opal
Boulder matrix opal
Yowah nuts
Boulder splits
Seam and vein opal
Sandstone opal
Pipe opal
Wood opal

Legends, Myths and Healing Properties:

OpalsAs indicated by the derivation of its name, Opal has centuries of history as a treasured gemstone. Historically, beliefs associated with the wearing of Opal have varied. The early Greeks thought that Opals gave their owners the powers of foresight and prophecy. Romans adored it as a token of hope and purity. Eastern people regarded it as sacred. Arabs believed it fell from heaven. In the nineteenth century, superstitions grew about the bad luck or fate that could befall one for wearing Opal if it were not the wearer's birthstone. Today, these superstitions have diminished, but some people still believe it is bad luck to wear Opals. Other legendary properties include: aids inner beauty, faithfulness, and eyesight and helps recall past lives.



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Smithsonian Institute 1998-2005 Bernardine Fine Art Jewelry. 1999-2004 Jewelry Central, JC Store.
2004 Amethyst Galleries, Inc. 1995-2005 Loretta Elaine's Gems for Friends.