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Opal, being amorphous, does not form crystals, and is generally found as layers or nodules within igneous or sedimentary rock. Amorphous materials such as opal are deposited at low temperatures by ground water that is saturated with silica. The amorphous gel enters voids in the matrix rock, where it most likely solidifies quickly and then looses water over a long period of time. Micro-fractures can also be filled with secondary silica, forming thin lamellae during solidification. Volcanic opal, also known as "mountain opal," is a precious opal that occurs within igneous rock, found in cracks, fissures and pockets in basalt, rhyolite or other volcanic material. Volcanic opal tends to have a slightly higher water content (5% to 9%) than sedimentary opal, but sedimentary opal can be as high as 12% or more. The traditional theory of opal formation, which asserts that opal is caused by seepage and percolation of liquid silica gel over long time-scales, has been challenged by some contemporary researchers who believe that opal was formed more rapidly than originally thought. They believe that a process of ion exchange occurred within a slurry of alumina, feldspar and other minerals typically associated with clays. This slurry was then saturated with an electrolyte solution of acid or alkali that would have caused ionic migration to occur [3,4]. Play of Color and Contra Luz OpalThe term "play of color," also known as "POC," "flash," "iridescence" or "fire," it the ability to "flash" alternating colors as the stone is rotated. While solidifying, both common and precious opal can form arrays of microscopic silica spheres, but only the closely packed, uniform rows (hexagonal close packed, or HCP) of identically-sized spheres in precious opal can break the light into its component colors by interference and diffraction. These spheres range in diameter from 0.0004 mm to 0.0005 mm. Common opal always displays the same color when viewed from any angle. Different areas in the opal can have lattices of different sized spheres, and different orientations of those spheres/lattices. The space in-between the spheres can also alter the color. This permits the opal to have different areas (zones, or "play of pattern") of color that are sharply defined, and flash at different angles as the opal is rotated.
Opal that displays three or more dominant color flashes is referred to as a "multifire opal." Most varieties of opal show a "face-up" play-of-color when incident light comes from above, while other rare varieties known as contra luz opal show their play-of-color when light travels through the stone from behind. The term "opalescence" is commonly misused to describe opals play-of-color, but it should really be used to describe the milky translucence of the body color found in many opals. Opal is typically singly refractive, or anomalous double refractive, with no birefringence or pleochroism. Hydrophane Opal & Cachalong OpalHydrophane opal is a highly porous variety with a dull surface luster, that may not display any play-of-color until it is immersed in water. The porous cavities in hydrophane opal can interfere with light transmission, but when these cavities are filled with fluid, light is able to pass through the specimen without interference, although this effect varies with the opal. Some specimens may have play of color all the time, while other ones may have play of color either only saturated or only dry. Many whitish opals from Andamooka, South Australia, and Virgin Valley, Nevada, as well as Welo or Mezezo "chocolate opal" from Ethiopia (aka "Ethiopian desert opal") are examples of hydrophane opal. Due to their porosity, hydrophane opal can be easily dyed to darken their base color. Like the hydrophane opal, the cachalong opal is a highly porous, whitish material that dehydrates, and becomes increasingly transparent as it is wetted, but cachalong does not display a play of color/fire when wet. The color of cachalong opal can range from milky white to creamy white, bluish white or yellowish white. Some extremely beautiful opals from places Nevada and Ethiopia may be discounted by the gem trade because of potential stability problems and fine pieces can end up being kept in water as uncut specimens. Bolder OpalGenerally, precious bolder opal is a nodule or concentration that was deposited as a thin layer in the matrix of ironstone, basalt, sandstone, etc. is more stable than thicker examples of opal. These concentrations can occur as round or elongated ellipsoids, or as horizontal bands. Bolder opal from Queensland, Australia — one of the only known sources for true boulder opals — has gained in popularity and price because of its availability, display of colors and superior stability.
Brazil produces a fine hard opal that has a good reputation for stability, but not in very large amounts. High-quality bolder opal will have high brilliance and a darker body color, with highlights of blue, green, orange and red. Fire OpalMexico has a special place in the opal world. It produces both precious opal and common opal that is called "fire opal" because of its yellow to orange (aka arananjado opal) to red body color. Some specimens of fire opal can be faceted because of their high clarity, but most fire opal is somewhat cloudy. This cloudiness in precious opal, which comes from internal structures such as the balls that defract the light, is called opalescence. Some of the finest fire opal comes from Querétaro, Mexico Fossil Opal & Opalized Petrified WoodSo-called "fossil opal," "fossilized opal," or "opal in wood matrix" is created by pseudomorphic replacement, when the original carbon-based object (tree limb casts, pine cones, bark, animal material, etc.) decays and the void is replaced hydrated silica, creating a replica of the original wood structure. Notable sources for fossilized opal are Queensland, Australia and the Virgin Valley in Humboldt County, Nevada, which was created from 14-million-year-old petrified wood from the Luisian Stage (Miocene rocks found in the Pacific Coast region of North America).  Opal History & OccurrenceThe use of opal by humans dates back to the dawn of man. So-called Ethiopian "chocolate opal" was discovered by in a cave in Kenya by the famed archeologist and paleontologist Louis Leakey in 1939 [6]. Historically, opal was produce for hundreds of years at the Dubnik mines in Hungary, now present-day Slovakia and the Czech Republic. The opal produced in Hungary was found in igneous rock, and had relatively little play of color by today's standards. As Europeans began to explore the globe, opal from the "new world" became available. Places like Mexico, the United States (Nevada, Idaho and Oregon), Honduras and Brazil all produced beautiful precious opal, mostly volcanic in origin.
Fire opal mining in Mexico dates back several hundred years, and the main mining region is located near the town of Magdalena in Jalisco, Mexico, and later in the state of Querétaro. All of the original mining locations were lost during the Spanish Conquest, but in 1840, Sir José Maria Siurab rediscovered some of the original deposits at what is known as the Santa Maria Iris Opal Mine in Hacienda Esmeralda. The nation of Ethiopia, in north-eastern Africa also produces some beautiful specimens of volcanic opal. The hydrophane variety known as "chocolate opal (aka "hydrophane fire opal") is found at Yita Ridge, near Mezezo (the woredas, or district of Mafud Mezezo Mojana) in the Amhara Region of the Ethiopian highlands. The so-called "Wello Opal," also known colloquially as "Crystal Fire Opal" is found to the north of Ethiopia's capital, Addis Ababa, in the province of Wollo. Opal Mining in the Australian OutbackAustralia is truly the "land of opal," with over 90% of the world's gem-quality opal coming from Australia's NSW, South Australia, and Queensland territories. Opal was first discovered in Australia by George Cragg in 1888, in present-day Opalton, Queensland. The primary source for Australian opal is within sedimentary sandstone, shale, and limestone deposits, mainly on the southern edge of the Great Artesian Basin. 15 to 30 million years ago, during the tertiary period, the Great Artesian Basin was covered with a large inland sea. As the sea retreated, conditions were right for ground waters saturated in silica to replace other minerals, make fossils, and permeate the rock to form thin seams and nodules of precious opal, alone with a vast amount of potch. After the formation of the opal, a long period of desiccation occurred that helped make Australian opal probably the most stable in the world.
Many of Australia's famous opal mines such as Lightning Ridge and White Cliffs opal fields in New South Wales; Koroit opal field, Quilpie opal field, Yowah opal field, and Opalton in Paroo Shire, Southwest Queensland; and Andamooka and Coober Pedy opal fields in South Australia produce the finest gem-quality opal in the world. Opal mining in the outback regions of White Cliffs and Coober Pedy is a chaotic and disorganized affair. With no centralized oversight, miners spread out across the plain, digging holes at random, leaving behind a strange landscape of mounds resembling the surface of the moon (above, left). The satellite view of White Cliffs in NSW Australia tells the whole story. Freelance miners and tourists can "noodle" (aka "fossick," "fossicking" or "prospecting") for opals without a permit, but a permit is required for organized (mechanized) mining. Up to 90 percent of the opal found in these inhospitable regions is valueless material known as "potch."
Opal that is mined from Australia's Andamooka mine is considered to be some of the most durable and stable opal due to its low water content. Fire opal is found in Central America, South America and the United States (Idaho and Oregon), typically within cavities formed in volcanic lava flows. Opal Crystallography, Chemistry, Physical Properties
Opal's Optical Properties
Opal Color ClassificationPrecious opal is divided into numerous sub-groups depending on its body color, incorporating matrix, size/shape of the areas of color. Thin pieces of precious opal can also be attached to, laminated to, or backed by foreign material forming an opal "doublet," or glued between a clear glass cap and a foreign backing (potch, glass or ironstone) to form a "triplet." These are sometimes referred to as "assembled opals." Both of these last constructions can produce attractive durable gems, but are much less desirable than gems made from solid opal. Opal is typically cut into a cabochon, although highly transparent fire opals can be facetted. Opal's color varieties are classified as: "Black Opal" (black body color) which has three designations from dark to light (Black 1, Black 2, Black 3), "Semi-Black" of "Grey" (dark body color), "Light" (between semi-black and white), "White" (hydrophane opal with very light body), and the rare "pineapple" psuedomorphic opal (ikaite) from White Cliffs in NSW, Australia.
General opal classifications are: "Boulder Opal" which is opal on a dark ironstone matrix (typical occurrence: Quilpie, Queensland); "Crystal Opal" which is transparent to semi-transparent black, semi-black or white opal which has a play of color (typical occurrence: South Australia and New South Wales); "Fire Opal" or "Arananjado Opal" which is translucent to transparent, with yellow, orange, or red body color (typical occurrence: Querétaro, Magdalena Jalisco, Mexico); "Contra Luz Opal" which is a precious opal where that only displays a play of color when held up to the light (typical occurrence: Opal Butte, Oregon); "Matrix Opal" or "Mass Opal" which has thin seams of precious opal in an ironstone matrix (typical occurrence: Andamooka, South Australia). Opal Treatments, Enhancements & CareOpal is a very fragile and brittle material when compared to other gemstones, and can easily chip, crack or fracture when exposed to shock. Due to its lack of durability, opal is occasionally treated or impregnated with mineral oil, glycerin, plastic, or wax which acts as a bonding agent. The only downside of these treatments is that they can yellow over time, diminishing the stone's play of color. Black opal, or white hydrophane opal is occasionally treated with chemicals, smoke (carbon), or dye. Opal crazing can also be stabilized using a bonding agent such as Opticon epoxy resin which has a high refractive index. Being a hydrated stone of variable composition, opal can have a high water content, and this can lead to breakage from sudden changes in temperature (thermal shock), and potential problems when cleaning the stone with steam or ultrasonic cleaners. Long term storage of opal is more secure if the gems are kept in a sealed bag with a damp piece of cotton to prevent dehydration. Even with precautions, some precious opal is not stable and will crack and "craze" in time. The market tends to discount even beautiful opal from a location with a poor record of stability. Synthetic or Simulant OpalLaboratory created opal was first introduced by the French company 'Gilson Created Opal' in 1974. In a laboratory, the opal growth process takes 14 to 18 months. Laboratory created opal has no water content, unlike natural opal, making it significantly more durable. Gilson opals exhibit a strong surface fluorescence and a "lizard skin" structure. Chatham Created Opal is another high-quality opal simulant that is physically similar to natural opal. There is a newer jewelry-grade opal simulant that is actually a polymer-impregnated manufactured opal. This simulated opal material has an opal-like silica lattice impregnated with plastic to bond the material together. Lab-created opals are manufactured and/or distributed by Created Opal Co. in NM, ICE Industries in Downey CA, and Morion Company in Brighton MA. Opal Inclusion Library - Gilson Opals Lawsuits Ignite Furor Over Created Opal Morion Company - Polymer-Impregnated Opals
1. The History of Opal 2. American Opal Society 3. Allan W. Eckert The World of Opals 4. len Cram, Beautiful Opals, Australia's National Gem 5. Opal Cutting Tips - Treating Fractured Opals 6. David Federman, Ethiopian Opal 7. Gems & Gemology, Opal from Shewa Province, Ethiopia 8. Fossil Opalized Petrified Wood 9. Queretaro - History of the Mexican Opal 9. Yowah Koroit Boulder and Australian Opals | Copyright © 2012 AllAboutGemstones.com. All rights reserved. | | | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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: Opal
