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| | IBUonline can be another B2B move platform for helping SMEs to get more orders from international buyers. In case of forced entry the device triggers number of high decibel sounds; to draw a passing police vehicle or neighbours. Most from the MNC's and Corporate Companies, use Vo - IP System to convey with their clients. You can easily control cameras remotely as well as the DVR can detect motion to turn cameras off and on, thus saving disk space and power. For the first few days, post the theft, I used an extra i - Phone 3 we had in a drawer. All in the data is collated and saved around the same singular network, which makes it much easier to search for your correct date and times thus making your daily life much easier.<br><br> |
| {{redirect|Opals}}
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| {{merge from|Ethiopian welo opal|date=November 2013}}
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| {{Infobox mineral
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| | name = Opal
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| | category = [[Mineraloid]]
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| | boxwidth =
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| | boxbgcolor =
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| | image = 31ct Crystal Opal Pendent set in 14k gold with Green Diamond Accents.JPG
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| | imagesize =
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| | caption = 31ct Crystal Opal Cabochon Pendant set in 14k gold with Green Diamond Accents
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| | formula = [[Hydrate]]d [[silica]]. SiO<sub>2</sub>·''n''H<sub>2</sub>O
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| | molweight =
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| | color = Colorless, white, yellow, red, orange, green, brown, black, blue
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| | habit = Irregular veins, in masses, in nodules
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| | system = Amorphous<ref name="GRG" />
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| | twinning =
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| | cleavage = None<ref name="GRG" />
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| | fracture = [[Conchoidal]] to uneven<ref name="GRG" />
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| | mohs = 5.5–6<ref name="GRG" />
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| | luster = Subvitreous to waxy<ref name="GRG" />
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| | polish = Vitreous to resinous<ref name="GRG" />
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| | refractive = 1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43<ref name="GRG" />
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| | opticalprop = Single refractive, often anomalous double refractive due to strain<ref name="GRG" />
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| | birefringence = none<ref name="GRG" />
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| | dispersion =
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| | pleochroism = None<ref name="GRG" />
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| | fluorescence= ''black'' or ''white'' body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce; ''common opal'': inert to strong green or yellowish green in long and short wave, may phosphoresce; ''fire opal'': inert to moderate greenish brown in long and short wave, may phosphoresce.<ref name="GRG">[[Gemological Institute of America]], ''GIA Gem Reference Guide'' 1995, ISBN 0-87311-019-6</ref>
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| | absorption = green stones: 660nm, 470nm cutoff<ref name="GRG" />
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| | streak = White
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| | gravity = 2.15 (+.08, -.90)<ref name="GRG" />
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| | density = 2.09
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| | melt =
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| | fusibility =
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| | diagnostic = darkening upon heating
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| | solubility = hot [[saline water|saltwater]], [[base (chemistry)|bases]], [[methanol]], [[humic acid]], [[hydrofluoric acid]]
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| | diaphaneity = opaque, translucent, transparent
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| | other =
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| | references = <ref>{{cite web|url=http://webmineral.com/data/Opal.shtml |title=Opal |publisher=Webmineral |date= |accessdate=2011-10-08}}</ref><ref>{{cite web|url=http://mindat.org/min-3004.html |title=Opal |publisher=Mindat.org |date= |accessdate=2011-10-08}}</ref>
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| }}
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| '''Opal''' is a hydrated [[amorphous]] form of [[Silicon dioxide|silica]]; its [[water]] content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a [[mineraloid]], unlike the other crystalline forms of silica which are classed as [[mineral]]s. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of [[rock (geology)|rock]], being most commonly found with [[limonite]], [[sandstone]], [[rhyolite]], [[marl]] and [[basalt]]. Opal is the national gemstone of [[Australia]], which produces 97% of the world's supply.<ref>[http://www.itsanhonour.gov.au/symbols/gemstone.cfm Gemstone], It's an Hohour (Australian Government site)</ref> This includes the production of the state of [[South Australia]], which accounts for approximately 80% of the world's supply.<ref>[http://www.premcab.sa.gov.au/emblems/opals1.htm Government of South Australia >Insignia and Emblems > Opal – South Australia's Gemstone] Accessed 11 July 2012.</ref>
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| The internal structure of precious opal makes it [[diffraction|diffract]] light; depending on the conditions in which it formed, it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. It varies in optical density from opaque to semi-transparent.
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| Common opal, called "potch" by miners, does not show the display of color exhibited in precious opal.<ref>[http://www.opalsdownunder.com.au/learn-about-opals/advanced/how-opal-formed-geology-opal]</ref>
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| == Precious opal ==
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| [[File:Opal molecular structure2.jpg|thumb|left|Precious opal consists of spheres of silicon dioxide molecules arranged in regular, closely packed planes. (Idealized diagram)]]
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| [[File:Coober Pedy Opal 2.jpg|thumb|left|Multicolor Rough Crystal Opal from Coober Pedy, South Australia, expressing nearly every color of the visible spectrum.]]
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| [[File:8ct Lightning Ridge Black Opal.JPG|thumb|left|Opal value is dependent on Color, Pattern, Brightness, Directionality, Base Color, and Size.<ref>Downing, Paul B.,Ph.D "Opal Identification and Value", 1992, pp. 113</ref>]]
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| [[File:10 7cts Brazilian Crystal Opal.jpg|thumb|left|Brightness of the fire in Opal ranges on a scale of 1 to 5 (5 being the brightest)<ref>Downing, Paul B.,Ph.D "Opal Identification and Value", 1992, pp. 55-61.</ref>]]
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| [[File:Opal Doublet.jpg|thumb|Australian Opal Doublet, a slice of precious opal with a backing of ironstone.]]
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| Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it has an internal structure. At micro scales precious opal is composed of silica spheres some 150 to 300 [[nanometre|nm]] in diameter in a hexagonal or cubic [[close-packing|close-packed]] [[crystal structure|lattice]]. These ordered silica spheres produce the internal colors by causing the [[interference (wave propagation)|interference]] and [[diffraction]] of light passing through the microstructure of the opal.<ref name=Klein>Klein, Cornelis, and Hurlbut, Cornelius S.; 1985, ''Manual of Mineralogy'', 20th ed., ISBN 0-471-80580-7</ref> It is the regularity of the sizes and 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 [[Diffraction grating|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.
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| 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|play of color]]''. Contrarily, ''opalescence'' is correctly applied to the milky, [[turbidity|turbid]] appearance of common or ''potch'' opal. Potch does not show a play of color.
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| For [[gemstone]] use, most opal is cut and polished to form a [[cabochon]]. "Solid" opal refers to polished stones consisting wholly of precious opal. Opals too thin to produce a "solid," may be combined with other materials to form attractive gems. An opal doublet consists of a relatively thin layer of precious opal, backed by a layer of dark-colored material, most commonly ironstone, dark or black common opal (potch), onyx or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch. An opal triplet is similar to a doublet, but has a third layer, a domed cap of clear [[quartz]] or plastic on the top. The cap takes a high polish and acts as a protective layer for the opal. The top layer also acts as a magnifier, to emphasize the play of color of the opal beneath, which is often of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal.
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| Combined with modern techniques of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.
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| == Common opal ==
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| [[File:MilkyRawOpal.jpg|thumb|left|A piece of milky raw opal from Andamooka South Australia]]
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| [[File:Common Opal (Opalized Wood).png|thumb|left|A cabochon cut from a piece of opalized wood]]
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| [[File:OpalDoublet.jpg|thumb|lright|An opal "triplet" from Andamooka South Australia showing blue and green fire]]
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| [[File:AndamookaOpalRock.jpg|thumb|left|A rock showing striations of opal throughout]]
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| [[File:OpalStriations.jpg|thumb|right|A close-up view of striations within opal]]
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| 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, which is honey-yellow with a resinous luster; [[wood opal]], which is caused by the replacement of the organic material in [[wood]] with opal;<ref>{{cite book|last=Gribble|first=C. D.|title=Rutley's Elements of Mineralogy|publisher=Unwin Hyman|location=London|year=1988|edition=27th ed.|page=431|chapter=Tektosilicates (framework silicates)|isbn=0-04-549011-2}}</ref> [[menilite]], which is brown or grey; [[hyalite]], a colorless glass-clear opal sometimes called Muller's Glass; [[geyserite]], also called [[siliceous sinter]], deposited around [[hot springs]] or [[geyser]]s; and [[diatomite]] or [[diatomaceous earth]], the accumulations of [[diatom]] shells or tests.
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| == Other varieties of opal ==
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| [[File:Opledefeu2.jpg|thumb|Fire opal from Mexico]]
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| '''Fire opal''' is a transparent to translucent opal, with warm body colors of yellow, orange, orange-yellow or red. It does not usually show any play of color, although occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of [[Querétaro]] in Mexico; these opals are commonly called Mexican fire opals. Fire opals that do not show play of color are sometimes referred to as jelly opals. Mexican opals are sometimes cut in their ryholitic host material if it is hard enough to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera Opal. There is also a type of opal from Mexico referred to as Mexican Water Opal, which is a colorless opal which exhibits either a bluish or golden internal sheen.<ref name="LetsTalk">James Swisher, Gemologist, and Edna B. Anthony, Gemologist, [http://www.attawaygems.com/NMFG/Lets_talk_gemstones_opal.htm Let's Talk Gemstones: Opal]</ref>
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| '''Girasol opal''' is a term sometimes mistakenly and improperly used to refer to fire opals as well as a type of transparent to semi-transparent type milky quartz from Madagascar which displays an asterism, or star effect, when cut properly. However, there is a true girasol opal<ref name="LetsTalk" /> that is a type of halite opal, that exhibits a bluish glow or sheen that follows the light source around. It is not a play of color as seen in precious opal but rather an effect from microscopic inclusions. It is also sometimes referred to as water opal as well when it is from Mexico. The two most notable locations of this type of opal are Oregon and Mexico.{{citation needed|date=December 2012}}
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| '''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. Blue opal also comes from Oregon in the Owhyee region as well as from Nevada around Virgin Valley.{{citation needed|date=December 2012}}
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| == Sources of opal ==
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| [[File:62cts Brazilian Crystal Opal.JPG|thumb|left|Gem Opal from Brazil]]
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| [[File:Opal from Yowah, Queensland, Australia 2.jpg|thumb|Polished opal from Yowah (Yowah Nut<ref name="Yowah nut">[http://www.mindat.org/min-9838.html Yowah Nut: Yowah Nut mineral information and data]. Mindat.org (2011-02-20). Retrieved on 2011-03-08.</ref>), Queensland, Australia]]
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| [[File:16.42cts Lightning Ridge black Opal.JPG|thumb|left|Multi-Colored solid black opal cabochon from Lightning Ridge, NSW]]
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| Australia produces around 97% of the world's opal. 90% is called 'light opal' or white and ''crystal opal''. White makes up 60% of the opal productions but cannot be found in all of the opal fields. ''Crystal opal'' or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.{{Citation needed|date=October 2007}}
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| The town of [[Coober Pedy, South Australia|Coober Pedy]] in [[South Australia]] is a major source of opal. The world's largest and most valuable gem opal "Olympic Australis" was found in {{Nowrap|August 1956}} at the "Eight Mile" opal field in Coober Pedy. It weighs 17,000 carats (3450 grams) and is {{convert|11|in|mm}} long, with a height of {{convert|4+3/4|in|mm}} and a width of {{convert|4+1/2|in|mm}}.{{citation needed|date=December 2012}}
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| [[File:Boulder Opal.jpg|thumb|right|200px| Boulder Opal, Carisbrooke Station near Winton, Queensland]]
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| The [[Mintabie, South Australia|Mintabie Opal Field]] located approximately 250 km north west of [[Coober Pedy]] has also produced large quantities of ''crystal opal'' and also the rarer ''black opal''. Over the years it has been sold overseas incorrectly as Coober Pedy Opal. The ''black opal'' is said to be some of the best examples found in Australia.
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| [[Andamooka, South Australia|Andamooka]] in South Australia is also a major producer of ''matrix opal'', ''crystal opal'', and ''black opal''. Another Australian town, [[Lightning Ridge, New South Wales|Lightning Ridge]] 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|Koroit]] in the south.<ref>[http://www.nrw.qld.gov.au/mines/fossicking/opal.html Queensland opal]{{Toter Link|date=October 2011}}</ref> Its largest quantities are found around [[Jundah]] and [[Quilpie]] (known as the "home of the Boulder Opal"<ref>Quilpie Shire Council "Simply Unique" 2010. page 8</ref>) in [[South West Queensland]]. Australia also has opalised fossil remains, including dinosaur bones in New South Wales, and marine creatures in South Australia.{{citation needed|date=December 2012}} The rarest type of Australian opal is "pipe" opal, closely related to ''boulder opal'', which forms in sandstone with some iron-oxide content, usually as fossilized tree roots.{{citation needed|date=December 2012}}
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| [[File:nev opal09.jpg|thumb|Multi-colored rough opal specimen from Virgin Valley, Nevada, US]]
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| The Virgin Valley<ref>[http://www.mindat.org/loc-5881.html Virgin Valley District, Humboldt Co., Nevada], mindat.org</ref> opal fields of [[Humboldt County, Nevada|Humboldt County]] in northern [[Nevada]] produce a wide variety of precious ''black, crystal, white, fire'', and ''lemon opal''. The ''black fire opal'' is the official gemstone of Nevada. Most of the ''precious opal'' is partial wood replacement. The ''precious opal'' is hosted and found within a subsurface horizon or zone of [[bentonite]] in-place which is considered a "lode" deposit. Opals which have weathered out of the in-place deposits are alluvial and considered [[placer]] deposits. [[Miocene]] age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest producing mines of Virgin Valley have been the famous Rainbow Ridge,<ref>[http://www.mindat.org/loc-19286.html Rainbow Ridge Mine], mindat.org</ref> Royal Peacock,<ref>[http://www.mindat.org/loc-30709.html Royal Peacock Group Mines, Virgin Valley District, Humboldt Co., Nevada], mindat.org</ref> Bonanza,<ref>[http://www.mindat.org/loc-8801.html Bonanza Opal Workings (Virgin Opal), Virgin Valley District, Humboldt Co., Nevada], mindat.org</ref> Opal Queen,<ref>[http://www.mindat.org/loc-213530.html Opal Queen group, Virgin Valley District, Humboldt Co., Nevada], mindat.org</ref> and WRT Stonetree/Black Beauty<ref>[http://www.mindat.org/loc-228982.html Stonetree Opal Mine, WRT Stonetree group, Virgin Valley District, Humboldt Co., Nevada], mindat.org</ref> Mines. The largest unpolished Black Opal in the Smithsonian Institution, known as the "Roebling Opal,"<ref>[http://geogallery.si.edu/index.php/en/1167897/roebling-opal/1 Roebling Opal], National Museum of Natural History</ref> came out of the tunneled portion of the Rainbow Ridge Mine in 1917, and weighs 2,585 carats. The largest polished black opal in the [[Smithsonian Institution]] comes from the Royal Peacock opal mine in the Virgin Valley, weighing 160 carats, known as the "Black Peacock."{{Citation needed|date=August 2008}}
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| Another source of ''white base opal'' or ''creamy opal'' in the United States is [[Spencer, Idaho]].{{Citation needed|date=July 2010}} A high percentage of the opal found there occurs in thin layers.
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| Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil (in [[Pedro II, Piauí]]<ref>{{cite web|url=http://www.mindat.org/gallery.php?loc=8782 |title=Boi Morto Mine, Pedro II, Piauí, Brazil |publisher=Mindat.org |date= |accessdate=2011-10-08}}</ref>), Honduras, Guatemala, Nicaragua and Ethiopia.
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| In late 2008, [[NASA]] announced that it had discovered opal deposits on [[Mars]].<ref>{{cite web|url=http://www.jpl.nasa.gov/news/news.cfm?release=2008-198|title=NASA probe finds opals in Martian crevices|accessdate=2008-10-29}}</ref>
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| == Synthetic opal ==
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| 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.<ref name=Klein /> 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. Furthermore, synthetic opals do not [[fluorescence|fluoresce]] under [[Ultraviolet|UV]] light. Synthetics are also generally lower in density and are often highly porous.
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| Two notable producers of synthetic opal are the companies [[Kyocera]] and [[Inamori]] of Japan. Most so-called synthetics, however, are more correctly termed "imitation opal", as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often foiled glass, glass-based "[[Slocum stone]]", or later plastic materials.
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| Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.<ref>{{cite web|url=http://www.license.umn.edu/Products/Highly-Ordered-Macroporous-Structures__99014.aspx |title=Macroporous Structures, Metal Oxides, Highly_Ordered – Office for Technology Commercialization, Express_license, University_of_Minnesota, Technology_Marketing_Site |publisher=License.umn.edu |date=2010-06-25 |accessdate=2011-10-08}}</ref>
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| == Local atomic structure of opals ==
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| 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 show no signs of crystalline order ([[amorphous]] opal) and those that show signs of the beginning of crystalline order, commonly termed [[cryptocrystalline]] or microcrystalline opal.<ref>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, Connecticut Prewitt, GV Gibbs, Mineralogical Society of America</ref> Dehydration experiments and [[infrared spectroscopy]] have shown that most of the H<sub>2</sub>O in the formula of SiO<sub>2</sub>·nH<sub>2</sub>O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and [[silanol]]s, 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.
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| The structure of low-pressure polymorphs of anhydrous [[silica]] consist of frameworks of fully corner bonded tetrahedra of SiO<sub>4</sub>. 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.
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| [[File:Alphacrist.png|thumb|The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.]]
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| === Microcrystalline opal ===
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| [[File:Lussatite-France.jpg|thumb|left|Lussatite (Opale-Ct)]]
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| 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, also called [[Lussatine]], is interpreted as consisting of localized order of <math>\alpha</math>-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.
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| === Non-crystalline opal ===
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| Two broad categories of non-crystalline opals, sometimes just referred to as "opal-A", have been proposed. The first of these is opal-AG consisting of 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. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. [[Hyalite]] is another name for this.
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| 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.<ref>{{cite web|url=http://minsocam.org/msa/AmMin/TOC/Articles_Free/1996/Cady_p1380-1395_96.pdf |title=(Cady ''et al.'', 1996) |format=PDF |date= |accessdate=2011-10-08}}</ref>
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| == Naming ==
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| The word ''opal'' is adapted from the Roman term ''opalus'', but the origin of this word is a matter of debate. However, most modern references suggest it is adapted from the [[Sanskrit]] word ''úpala.<ref name=eckert>Allan Eckert, ''[http://books.google.ca/books?id=vGewAkLkmvIC&pg=PA56 The World of Opals]'', John Wiley and Sons, 1997, pp. 56–57</ref>
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| References to the gem are made by [[Pliny the Elder]]. It is suggested it was adapted it from [[Ops]], the wife of [[Saturn (mythology)|Saturn]] and goddess of fertility. The portion of [[Saturnalia]] devoted to Ops was "Opalia", similar to opalus.
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| Another common claim that the term is adapted from the Greek word, ''opallios''. This word has two meanings, one is related to "seeing" and forms the basis of the English words like "opaque", the other is "other" as in "alias" and "alter". It is claimed that opalus combined these uses, meaning "to see a change in color". However, historians have noted that the first appearances of ''opallios'' do not occur until after the Romans had taken over the Greek states in 180 BC, and they had previously used the term ''paederos''.<ref name=eckert />
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| However, the argument for the Sanskrit origin is strong. The term first appears in Roman references around 250 BC, at a time when the opal was valued above all other gems. The opals were supplied by traders from the [[Bosporus]], who claimed the gems were being supplied from India. Before this the stone was referred to by a variety of names, but these fell from use after 250 BC.
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| == Historical superstitions ==
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| 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.<ref name=Fernie>{{cite book | last = Fernie | first = William Thomas | title = Precious Stones for Curative Wear | publisher=Bristol, John Wright & Co | year= 907| pages=248–249 |url=http://books.google.com/books?id=-wMAAAAAYAAJ&printsec=frontcover#v=onepage&q&f=false}}</ref> It was also said to confer the power of invisibility if wrapped in a fresh [[bay leaf]] and held in the hand.<ref name=Fernie /><ref>{{cite book|title=Wicca Candle Magick|first=Gerina|last=Dunwich |author-link=Gerina Dunwich |year=1996|pages=84–85}}</ref> Following the publication of Sir [[Walter Scott]]'s ''[[Anne of Geierstein]]'' in 1829, however, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opal [[amulet|talisman]] with supernatural powers. When a drop of [[holy water]] falls on the talisman, the opal turns into a colorless stone and the Baroness dies soon thereafter. Due to the popularity of Scott's novel, people began to associate opals with bad luck and death.<ref name=Fernie /> Within a year of the publishing of Scott's novel in April 1829, the sale of opals in Europe dropped by 50%, and remained low for the next twenty years or so.<ref>[[Allan W. Eckert|Eckert, Allan W.]] "A Chronological History and Mythology of Opals." In: ''The World of Opals''. New York: John Wiley & Sons, 1997. See pp. 53-118.</ref>
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| Even as recently as the beginning of the 20th century, it was believed that when a Russian saw an opal among other goods offered for sale, he or she should not buy anything more since the opal was believed to embody the [[evil eye]].<ref name=Fernie />
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| Opal is considered the [[birthstone]] for people born in October or under the sign of [[Scorpio (astrology)|Scorpio]] and [[Libra (astrology)|Libra]].
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| == Famous opals ==
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| * The [[Olympic Australis]], the world's largest and most valuable gem opal{{citation needed|date=December 2012}}
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| * The [[Andamooka Opal]], presented to [[Elizabeth II|Queen Elizabeth II]], also known as the Queen's Opal
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| * The Burning of Troy, the now-lost opal presented to [[Joséphine de Beauharnais]] by [[Napoleon I of France]] and the first named opal<ref>{{cite book |author=Eckert, Allan W. |title=The World of Opals |publisher=John Wiley & Sons |location=Chichester |year=1997 |pages=67, 126 |isbn=0-471-13397-3 }}</ref>
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| * The [[Flame Queen Opal]]
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| * The [[Halley's Comet Opal]], the world's largest uncut black opal
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| * Although the clock faces above the information stand in [[Grand Central Terminal]] Manhattan, New York, are often said to be opal, they are in fact opalescent glass
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| * The Roebling Opal, Smithsonian Institution<ref>{{cite web|url=http://www.mnh.si.edu/earth/text/dynamicearth/6_0_0_GeoGallery/geogallery_specimen.cfm?SpecimenID=4038&categoryID=1&categoryName=Gems&browseType=name |title=The Dynamic Earth @ National Museum of Natural History |publisher=Mnh.si.edu |date= |accessdate=2011-10-08}}</ref>
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| * The [[Galaxy Opal]], listed as the "World's Largest Polished Opal" in the 1992 Guinness Book of Records<ref>"The Guinness book of records, 1993" October 1992, ISBN 0-85112-978-1, [http://books.google.com/books?id=oqoOYFaIIigC&dq=galaxy+opal&q=galaxy+opal+#search_anchor p.22]</ref>
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| == See also ==
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| {{Wikisource|1911 Encyclopædia Britannica/Opal}}
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| * [[Opalite]]
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| * [[Optical phenomena]]
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| == References ==
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| {{Reflist|30em}}
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| == External links ==
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| {{Commons category|Opal}}
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| {{Wiktionary|opal}}
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| * [http://www.farlang.com/gemstones/opal Farlang opal Hist. References] Localities, anecdotes by [[Theophrastus]], [[Isaac Newton]], [[Georg Agricola]] etc.
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| * [http://gemstone.org/gem-by-gem/english/opal.html ICA's Opal Page]: International Colored Stone Association
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| * [http://www.samuseum.sa.gov.au/whatson/galleries/fossils South Australian Museum > The Opal Fossils of South Australia] Accessed 2 December 2011.
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| {{Jewellery}}
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| {{Silica minerals}}
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| [[Category:Gemstones]]
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| [[Category:Glass in nature]]
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| [[Category:Hydrates]]
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| [[Category:National symbols of Australia]]
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| [[Category:Emblems of South Australia]]
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| [[Category:Quartz varieties]]
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