Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Opal shopping experience:
1. Compare - without doubt the biggest advantage that the Opal offers shoppers today is the ability to compare thousands of Opal at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.
2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about
3. Testimonials - don't know anybody that has bought a Opal? Wrong! If the Opal is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.
4. Questions - Got a question about Opal then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....
5. Reputation - Never heard of the company selling Opal? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Opal and build up a picture of their reputation for sales, returns, customer service, delivery etc.
6. Returns - still worried that even after all of the above your Opal wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.
7. Feedback - happy with your Opal then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.
8. Security - check for the yellow padlock on the Opal site before you buy, and the s after http:/ /i.e. https:// = a secure site
9. Contact - got a question about Opal, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.
10. Payment - ready to pay for your Opal, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.
{{Infobox mineral| name = Opal| category =
Mineraloidd [silica. SiO2
·nH2O]. Very infrequently of a singular color| habit = Irregular veins, in masses, in nodules| system = Amorphous| twinning =| cleavage = None| fracture = Conchoidal to uneven], GIA Gem Reference Guide 1995, ISBN 0-87311-019-6| absorption = green stones: 660nm, 470nm cutoff| streak = White| gravity = 2.15 (+.08, -.90)| density =| melt =| fusibility =| diagnostic = darkening upon heating| solubility = hot saltwater, base (chemistry)s, methanol, humic acid, hydrofluoric acid opal is [amorphous SiO2·nH2O, hydrated silicon dioxide, the water content sometimes being as high as 20% but is usually between three and ten percent. Opal ranges from clear through white, gray, red, yellow, green, shore, blue, magenta, brown, and black. Of these hues, red and black are the most rare and dear, whereas white and green are the most common; these are a function of growth size into the red and infrared wavelengths—see #Precious opal. Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Latin opalus, by Greek language òpalliòs, and is from the same root as Sanskrit upálá for "stone", originally a millstone with upárá for slab. Sanskrit, Tamil, and Pahlavi Dictionaries. U. of Cologne. (see Upal). Opals are also Australia's national gemstone.
Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of Rock (geology), being most commonly found with limonite, sandstone, rhyolite, and basalt.
Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors.
Precious opal
Precious opal shows a variable interplay of internal colors and does have an internal structure. At the micro scale precious opal is composed of silica spheres some 150 to 300 nanometre in diameter in a hexagonal or cubic close-packing crystal structure. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of opal (Klein and Hurlbut, 1985, p. 444). It is the regularity of the sizes of the spheres, and of the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction. Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example.
In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed iridescence. Contrarily, opalescence is correctly applied to the milky, turbidity appearance of common or potch opal. Potch does not show a play of color.
The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet (lapidary) is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a domed cap of clear quartz (rock crystal) or plastic on top, which takes a high polish, and acts as a protective layer for the opal. Opal doublets and triplets are not classed as precious opal.
Common opal
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, honey-yellow with a resinous lustre; wood opal, caused by the replacement of the organic material in wood with opal; menilite brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Other varieties of opal
Fire opal, or Girasol, is a translucent to semi-opaque stone that is generally yellow to bright orange and sometimes nearly red and displays pleochroism at certain angles.
Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
Sources of opal
Until the nineteenth century the only source of precious opal known to Europeans was the mining district of Červenica in Slovakia.
Opal without play of color is very common and can be found all over the world, unlike precious opal deposits that are in greater scope found today only in Australia, U.S. and Mexico.
Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% but not all the opal fields produce white opal; Crystal opal or pure hydrated silica makes up 30%; 8% is black and only 2% is boulder opal.
The town of Coober Pedy, South Australia in South Australia is a major source of opal. Another Australian town, Lightning Ridge, New South Wales in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit opal field in the south.http://www.nrw.qld.gov.au/mines/fossicking/opal.html Queensland opal
Fire opal is found mostly in Mexico and Mesoamerica. In South America opal discovered in 1930 in a city called Pedro II in Brazil is still produced. In Honduras there was also some fine black opal mined from volcanic ash deposits. This opal is known for its stability.
The Virgin Valley Opal Fields Of Humboldt County in Northern Nevada produce a wide variety black, crystal, white, and fire opal.The Fire Opal mined from this locality is designated as the official gemstone of the State of Nevada. Most precious opals are wood replacements. Many specimens have a high water content, and as a result, have a greater tendency to desiccate and crack than most precious opal. Discovered in 1904 the mines are still producing gem materials in large amounts to hundreds of seasonal visitors. Three Fee Dig Mines provide the general public an opportunity to dig the gems themselves. The largest black opal in the Smithsonian Museum, possibly worth in excess of $1 million, comes from the Royal Peacock Opal Mine in the Virgin Valley.
Another source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.
Synthetic opal
As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974 (Klein and Hurlbut, 1985, p.528). The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue.
Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewellery are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.
The Hamamatsu Photonics Group will utilise the International Space Station's Japanese Experiment Module in 2006 to grow perfect crystalline opals in microgravity over four months, as compared with five million years for naturals. Such opals will be the object of study and elements for optical filters, displays, and data storage."". Science & Technology in Japan No. 85. (2005).
- The gem-quality opal is a color-sensitive mirror called a photonic crystal. 27th Space Station Utilization Workshop in Japan -outline -
Local atomic structure of opals
The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that have show no signs of crystalline order, i.e., amorphous opal, and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal (Graetsch, 1994). Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2.nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.
The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
Microcrystalline opal
Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym.
Opal-C is interpreted as consisting of localized order of \alpha-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%. Lussatine is a synonym.
Non-crystalline opal
Two broad categories of non-crystalline opals, sometimes just referred to as opal-A, have been proposed
Opal-AG: Aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing.
Opal-AN: Water-containing amorphous silica-glass. Hyalite is a synonym.
Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed .
Historical superstitions
In the middle ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. , cpages 249 Even under the last czar at the beginning of the 20th century, it was believed that when a Russian of any sex, of any rank, saw an opal, amongst other goods offered for sale, he or she would not buy anything more, since, in the judgement of subjects of the czar, the opal embodied the evil eye.
Opals in popular culture
The opal is the official gemstone of South Australia and the Australia, and the country's Australia women's national basketball team in basketball is nicknamed The Opals.
The official state gem stone for Nevada is precious black Fire Opal, in recognition of the black opal found in Virgin Valley, Humboldt County, Nevada.
Opal is the traditional birthstone of the month of October.
Famous opals
- The Andamooka Opal, presented to Queen Elizabeth II, also known as the Queen's Opal
- The Aurora Australis Opal, considered to be the most valuable black opal
- The Black Prince Opal, originally known as Harlequin Prince
- The Empress of Australia Opal
- The Fire Queen Opal
- The Flame Queen Opal
- The Flamingo Opal
- The Halley's Comet Opal, the world's largest uncut black opal
- The Jupiter Five Opal
- The Olympic Australis Opal, reported to be the largest and most valuable gem opal ever found
- The Pride of Australia Opal, also known as the Red Emperor Opal
- The Red Admiral Opal, also known as the Butterfly Stone
See also
References
- Graetsch, H., "Structural Characteristics of opaline and microcrystalline silica minerals", "Silica, physical behavior, geochemistry and materials applications". Reviews in Mineralogy, Vol. 29, 1994. Editors PJ Heaney, CT Prewitt, GV Gibbs, Mineralogical Society of America,
- Klein, Cornelis, and Hurlbut, Cornelius S.; 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
- Mindat.org: Opal: Precious Opal Gallery, Opal Gallery.
- Berkeley—Great photos
- South Australian Government Floral Emblem (official website)
- http://gemstone.org/gem-by-gem/english/opal.html
- http://webmineral.com/data/Opal.shtml
- http://mineral.galleries.com/minerals/mineralo/opal/opal.htm
{{Infobox mineral| name = Opal| category = Mineraloidd [silica. SiO2·nH2O]. Very infrequently of a singular color| habit = Irregular veins, in masses, in nodules| system = Amorphous| twinning =| cleavage = None| fracture = Conchoidal to uneven], GIA Gem Reference Guide 1995, ISBN 0-87311-019-6| absorption = green stones: 660nm, 470nm cutoff| streak = White| gravity = 2.15 (+.08, -.90)| density =| melt =| fusibility =| diagnostic = darkening upon heating| solubility = hot saltwater, base (chemistry)s, methanol, humic acid, hydrofluoric acid opal is [amorphous SiO2·nH2O, hydrated silicon dioxide, the water content sometimes being as high as 20% but is usually between three and ten percent. Opal ranges from clear through white, gray, red, yellow, green, shore, blue, magenta, brown, and black. Of these hues, red and black are the most rare and dear, whereas white and green are the most common; these are a function of growth size into the red and infrared wavelengths—see #Precious opal. Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Latin opalus, by Greek language òpalliòs, and is from the same root as Sanskrit upálá for "stone", originally a millstone with upárá for slab. Sanskrit, Tamil, and Pahlavi Dictionaries. U. of Cologne. (see Upal). Opals are also Australia's national gemstone.
Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of Rock (geology), being most commonly found with limonite, sandstone, rhyolite, and basalt.
Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors.
Precious opal
Precious opal shows a variable interplay of internal colors and does have an internal structure. At the micro scale precious opal is composed of silica spheres some 150 to 300 nanometre in diameter in a hexagonal or cubic close-packing crystal structure. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of opal (Klein and Hurlbut, 1985, p. 444). It is the regularity of the sizes of the spheres, and of the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction. Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example.
In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed iridescence. Contrarily, opalescence is correctly applied to the milky, turbidity appearance of common or potch opal. Potch does not show a play of color.
The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet (lapidary) is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. Given the texture of opals, they can be quite difficult to polish to a reasonable lustre. The triplet cut backs the colored material with a dark backing, and then has a domed cap of clear quartz (rock crystal) or plastic on top, which takes a high polish, and acts as a protective layer for the opal. Opal doublets and triplets are not classed as precious opal.
Common opal
Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality); resin opal, honey-yellow with a resinous lustre; wood opal, caused by the replacement of the organic material in wood with opal; menilite brown or grey; hyalite, a colorless glass-clear opal sometimes called Muller's Glass; geyserite, (siliceous sinter) deposited around hot springs or geysers; and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Other varieties of opal
Fire opal, or Girasol, is a translucent to semi-opaque stone that is generally yellow to bright orange and sometimes nearly red and displays pleochroism at certain angles.
Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.
Sources of opal
Until the nineteenth century the only source of precious opal known to Europeans was the mining district of Červenica in Slovakia.
Opal without play of color is very common and can be found all over the world, unlike precious opal deposits that are in greater scope found today only in Australia, U.S. and Mexico.
Australia produces around 97% of the world’s opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% but not all the opal fields produce white opal; Crystal opal or pure hydrated silica makes up 30%; 8% is black and only 2% is boulder opal.
The town of Coober Pedy, South Australia in South Australia is a major source of opal. Another Australian town, Lightning Ridge, New South Wales in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit opal field in the south.http://www.nrw.qld.gov.au/mines/fossicking/opal.html Queensland opal
Fire opal is found mostly in Mexico and Mesoamerica. In South America opal discovered in 1930 in a city called Pedro II in Brazil is still produced. In Honduras there was also some fine black opal mined from volcanic ash deposits. This opal is known for its stability.
The Virgin Valley Opal Fields Of Humboldt County in Northern Nevada produce a wide variety black, crystal, white, and fire opal.The Fire Opal mined from this locality is designated as the official gemstone of the State of Nevada. Most precious opals are wood replacements. Many specimens have a high water content, and as a result, have a greater tendency to desiccate and crack than most precious opal. Discovered in 1904 the mines are still producing gem materials in large amounts to hundreds of seasonal visitors. Three Fee Dig Mines provide the general public an opportunity to dig the gems themselves. The largest black opal in the Smithsonian Museum, possibly worth in excess of $1 million, comes from the Royal Peacock Opal Mine in the Virgin Valley.
Another source of white base opal in the United States is Spencer, Idaho. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.
Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, Nicaragua and Ethiopia.
Synthetic opal
As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974 (Klein and Hurlbut, 1985, p.528). The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous; some may even stick to the tongue.
Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed imitations, as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewellery are often "Slocum Stone" consisting of laminated glass with bits of foil interspersed.
The Hamamatsu Photonics Group will utilise the International Space Station's Japanese Experiment Module in 2006 to grow perfect crystalline opals in microgravity over four months, as compared with five million years for naturals. Such opals will be the object of study and elements for optical filters, displays, and data storage."". Science & Technology in Japan No. 85. (2005).
- The gem-quality opal is a color-sensitive mirror called a photonic crystal. 27th Space Station Utilization Workshop in Japan -outline -
Local atomic structure of opals
The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that have show no signs of crystalline order, i.e., amorphous opal, and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal (Graetsch, 1994). Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2.nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.
The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
Microcrystalline opal
Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym.
Opal-C is interpreted as consisting of localized order of \alpha-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%. Lussatine is a synonym.
Non-crystalline opal
Two broad categories of non-crystalline opals, sometimes just referred to as opal-A, have been proposed
Opal-AG: Aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing.
Opal-AN: Water-containing amorphous silica-glass. Hyalite is a synonym.
Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed .
Historical superstitions
In the middle ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal. , cpages 249 Even under the last czar at the beginning of the 20th century, it was believed that when a Russian of any sex, of any rank, saw an opal, amongst other goods offered for sale, he or she would not buy anything more, since, in the judgement of subjects of the czar, the opal embodied the evil eye.
Opals in popular culture
The opal is the official gemstone of South Australia and the Australia, and the country's Australia women's national basketball team in basketball is nicknamed The Opals.
The official state gem stone for Nevada is precious black Fire Opal, in recognition of the black opal found in Virgin Valley, Humboldt County, Nevada.
Opal is the traditional birthstone of the month of October.
Famous opals
See also
References
- Graetsch, H., "Structural Characteristics of opaline and microcrystalline silica minerals", "Silica, physical behavior, geochemistry and materials applications". Reviews in Mineralogy, Vol. 29, 1994. Editors PJ Heaney, CT Prewitt, GV Gibbs, Mineralogical Society of America,
- Klein, Cornelis, and Hurlbut, Cornelius S.; 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
- Mindat.org: Opal: Precious Opal Gallery, Opal Gallery.
- Berkeley—Great photos
- South Australian Government Floral Emblem (official website)
- http://gemstone.org/gem-by-gem/english/opal.html
- http://webmineral.com/data/Opal.shtml
- http://mineral.galleries.com/minerals/mineralo/opal/opal.htm
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