Opal 54513 226108833 2008-07-16T21:23:28Z SimpsonDG 2448001 /* Common opal */ Corrected title of reference {{alternateuses}} {{Infobox mineral | name = Opal | category = [[Mineraloid]] | boxwidth = | boxbgcolor = | image = Opal_Armband_800pix.jpg | imagesize = | caption = An opal bracelet. The stone size is 18 by 15 mm (0.7 by 0.6 inch). | formula = [[Hydrate]]d [[silica]]. SiO<sub>2</sub>'''·'''''n''H<sub>2</sub>O | molweight = | color = White, black, red, orange, most of the full spectrum, colorless, [[iridescent]]. Very infrequently of a singular color | habit = Irregular veins, in masses, in nodules | system = Amorphous<ref name="GRG"/> | twinning = | cleavage = None<ref name="GRG"/> | fracture = [[Conchoidal]] to uneven<ref name="GRG"/> | mohs = 5.5–6.5<ref name="GRG"/> | luster = Subvitreous to waxy<ref name="GRG"/> | polish = Vitreous to resinous<ref name="GRG"/> | refractive = 1.450 (+.020, -.080) Mexican Opal may read as low as 1.37, but typically reads 1.42–1.43<ref name="GRG"/> | opticalprop = Single refractive, often anomalous double refractive due to strain<ref name="GRG"/> | birefringence = none<ref name="GRG"/> | dispersion = | pleochroism = None<ref name="GRG"/> | 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> | absorption = green stones: 660nm, 470nm cutoff<ref name="GRG"/> | streak = White | gravity = 2.15 (+.08, -.90)<ref name="GRG"/> | density = | melt = | fusibility = | diagnostic = darkening upon heating | solubility = hot [[saltwater]], [[base (chemistry)|base]]s, [[methanol]], [[humic acid]], [[hydrofluoric acid]] | diaphaneity = opaque, translucent, transparent | other = }} '''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)|rock]], being most commonly found with [[limonite]], [[sandstone]], [[rhyolite]], and [[basalt]]. The water content is usually between three and ten percent, but can be as high as 20%. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare and dear, whereas white and greens are the most common; these are a function of growth size into the [[red]] and [[infrared]] wavelengths—see [[#Precious opal|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|Greek]] ''opallios'', and is from the same root as [[Sanskrit]] ''upálá[s]'' for "stone", originally a millstone with ''upárá[s]'' for slab.<ref name=San>[http://webapps.uni-koeln.de/tamil Sanskrit, Tamil, and Pahlavi Dictionaries]. U. of Cologne.</ref> (see [[Upal]]). Opals are also [[Australia]]'s national gemstone. Opal is one of the mineraloids that can form or replace [[fossil]]s. 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|nm]] in diameter in a hexagonal or cubic [[close-packing|closed-packed]] [[crystal structure|lattice]]. 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. [[Image:Close-packed spheres.jpg|thumb|left|180px|Precious opal consists of spheres of silica of fairly regular size, packed into close-packed planes which are stacked together with characteristic dimensions of several hundred nm.]] 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. The veins of opal displaying the play of color are often quite thin, and this has given rise to [http://opalsdownunder.com.au/articles/cutting.php unusual methods] of preparing the stone as a gem. An opal ''[[doublet (lapidary)|doublet]]'' is a thin layer of hueladen material, backed by a swart 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 gleam. 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 relatively fragile 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 luster; '''wood opal''', 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|date=1988|edition=27th ed.|pages=p. 431|chapter=Tektosilicates (framework silicates)|isbn=0045490112}}</ref>; [[menilite]] brown or grey; [[hyalite]], a colorless glass-clear opal sometimes called Muller's Glass; [[geyserite]], ([[siliceous sinter]]) deposited around [[hot springs]] or [[geyser]]s; and [[diatomite]] or [[diatomaceous earth]], the accumulations of [[diatom]] shells or tests. ==Other varieties of opal== '''Fire opal''' – Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not show any play-of-color. The most famous source of fire opals is Mexico and these opals are commonly called Mexican fire opals. '''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. [[Image:Opal Walros 800pix.jpg|thumb|left|250px|Boulder opal carving of a walrus, showing flashes of color from the exposed opal. The carving is 9 cm (3.5 inches) long.]] ==Sources of opal== 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., Mexico, Brazil, and Ethiopia. [[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.{{Fact|date=October 2007}} The town of [[Coober Pedy, South Australia|Coober Pedy]] in [[South Australia]] is a major source of 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</ref> [[Image:nev opal09.jpg|thumb|right|250px|Multi-color rough opal specimen from Virgin Valley, Nevada, USA]] ''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.{{Fact|date=February 2007}} 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]].<ref>"{{PDFlink|[http://idb.exst.jaxa.jp/edata/02497/pdf/STJ85-09.pdf Utilization of "KIBO" in Nano-technology and Nano-science—Nature's Beauty to Be Reproduced in Space]|537&nbsp;[[Kibibyte|KiB]]<!-- application/pdf, 550334 bytes -->}}". ''Science & Technology in Japan'' No. 85. (2005). *The gem-quality opal is a color-sensitive mirror called a [[photonic crystal]].</ref><ref>[http://idb.exst.jaxa.jp/edata/02498/200603E02498000.html 27<sup>th</sup> Space Station Utilization Workshop in Japan -outline -]</ref> ==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 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 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. 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. [[Image:Alphacrist.jpg|thumb|The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.]] ===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 <math>\alpha</math>-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 {{PDFlink|[http://minsocam.org/msa/AmMin/TOC/Articles_Free/1996/Cady_p1380-1395_96.pdf (Cady ''et al.'', 1996)]|2.03&nbsp;[[Mebibyte|MiB]]<!-- application/pdf, 2137035 bytes -->}}. ==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. <ref name=Fernie>{{cite book | last = Fernie M.D. | first = W.T. | title = Precious Stones for Curative Wear | publisher = Bristol, John Wright & Co. | year = 1907}}, cpages 249 </ref> However, modern superstition attributes bad luck to the stone, though some believe this is avoided if opal is the owner's [[Birthdays#birthstones|birthstone]] (that is, the owner was born in October) or if the stone is a gift. 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 tzar, the opal embodied the evil eye. <ref name=Fernie/> It's possible that the stone's extreme fragility (when compared to other gemstones) has contributed to this bad reputation. ==Opals in popular culture== The opal is the official [[gemstone]] of [[South Australia]], The [[Australia|Commonwealth of Australia]] and [[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 uncut 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 == {{wikisourcepar|EB1911:Opal}} *[[Optical phenomena]] *[[Upal]] *[[Opalite]] ==References== {{Commonscat|Opal}} {{reflist}} *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 ==External links== *[http://www.rubymill.co.uk/geminfo/geminfopages/opal.html Ruby Mill's opal page] *[http://opalsdownunder.com.au/articles/famous.php Pictures and information on famous opals] *[http://mindat.org/min-3004.html Mindat.org: Opal]: [http://mindat.org/gallery.php?min=6666 Precious opal gallery], [http://mindat.org/gallery.php?min=3004 Opal gallery] *[http://www.farlang.com/gemstones/opal Farlang Opal Hist. References]: localities, anecdotes by [[Theophrastus]], [[Isaac Newton]], [[Georg Agricola]] etc. *[http://ist-socrates.berkeley.edu/~eps2/wisc/Lect16b.html Berkeley agates and opals]: Photos *[http://gemstone.org/gem-by-gem/english/opal.html ICA's Opal Page]: International Colored Stone Association *[http://webmineral.com/data/Opal.shtml Webmineral: Opal]: crystallographic and mineralogical info {{Jewellery Materials}} {{Silica minerals}} [[Category:Quartz varieties]] [[Category:Gemstones]] [[Category:Silicate minerals]] [[ar:العقيق (حجر)]] [[bs:Opal]] [[bg:Опал]] [[cs:Opál]] [[de:Opal]] [[et:Opaal]] [[es:Ópalo]] [[eo:Opalo]] [[eu:Opalo]] [[fr:Opale]] [[gl:Ópalo]] [[it:Opale]] [[he:אופאל]] [[la:Opalus]] [[lt:Opalas]] [[hu:Opál]] [[nl:Opaal]] [[ja:オパール]] [[no:Opal]] [[pl:Opal (minerał)]] [[pt:Opala]] [[ro:Opal]] [[ru:Опал]] [[simple:Opal]] [[sk:Opál]] [[sl:Opal]] [[fi:Opaali]] [[sv:Opal]] [[vi:Đá opal]] [[tr:Opal]] [[uk:Опал]] [[zh:蛋白石]]