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Opal

Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

Opal
A rich seam of iridescent opal encased in matrix
General
CategoryMineraloid
Formula
(repeating unit)
Hydrated silica. SiO2·nH2O
IMA symbolOpl[1]
Crystal systemAmorphous[2]
Identification
ColorColorless, white, yellow, red, orange, green, brown, black, blue, pink
Crystal habitIrregular veins, in masses, in nodules
CleavageNone[2]
FractureConchoidal to uneven[2]
Mohs scale hardness5.5–6[2]
LusterSubvitreous to waxy[2]
StreakWhite
Diaphaneityopaque, translucent, transparent
Specific gravity2.15+0.08
−0.90
[2]
Density2.09 g/cm3
Polish lusterVitreous to resinous[2]
Optical propertiesSingle refractive, often anomalous double refractive due to strain[2]
Refractive index1.450+0.020
−0.080

Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[2]
Birefringencenone[2]
PleochroismNone[2]
Ultraviolet fluorescenceblack 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[2]
Absorption spectragreen stones: 660 nm, 470 nm cutoff[2]
Diagnostic featuresdarkening upon heating
Solubilityhot salt water, bases, methanol, humic acid, hydrofluoric acid
References[3][4]
Main opal producing countries

The name opal is believed to be derived from the Sanskrit word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).

There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence); common opal does not.[5] Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."[6] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered the rarest, while white, gray, and green opals are the most common.

Precious opal edit

 
Precious opal consists of spheres of silica molecules arranged in regular, closely packed planes (idealized diagram).
 
This precious rough opal from Coober Pedy, South Australia, displays nearly every color of the visible spectrum.

Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At microscopic scales, precious opal is composed of silica spheres some 150–300 nm (5.9×10−6–1.18×10−5 in) in diameter in a hexagonal or cubic close-packed lattice. It was shown by J. V. Sanders in the mid-1960s[7][8] that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[9] The regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal. Where the distance between the regularly packed planes of spheres is around 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 colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light. The process can be described by Bragg's law of diffraction.

Visible light cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal. The notion that opals are photonic crystals for visible light was expressed in 1995 by Vasily Astratov's group.[10] In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during its formation. The term opalescence is commonly used to describe this unique and beautiful phenomenon, which in gemology is termed play of color. In gemology, opalescence is applied to the hazy-milky-turbid sheen of common or potch opal which does not show a play of color.[clarification needed] Opalescence is a form of adularescence.

For gemstone use, most opal is cut and polished to form a cabochon. "Natural" opal refers to polished stones consisting wholly of precious opal. Opals too thin to produce a "natural" opal may be combined with other materials to form "composite" 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 an inferior specimen or an extremely thin section of precious opal. Triplet opals tend to have a more artificial appearance and are not classed as precious gemstones, but rather "composite" gemstones. Jewelry applications of precious opal can be somewhat limited by opal's sensitivity to heat due primarily to its relatively high water content and predisposition to scratching.[11] Combined with modern techniques of polishing, a doublet opal can produce a similar effect to Natural black or boulder opal at a 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.

Common opal edit

 
Common rough opal
 
White and blue opal from Slovakia

Besides the gemstone varieties that show a play of color, the other kinds of common opal include 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;[12] 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 geysers; and diatomaceous earth, the accumulations of diatom shells or tests. Common opal often displays a hazy-milky-turbid sheen from within the stone. In gemology, this optical effect is strictly defined as opalescence which is a form of adularescence.

Varieties of Common Opal edit

 
Brazilian Precious Opal with patches of brilliant color throughout. The brightness of the fire in opal ranges on a scale of 1 to 5 (with 5 being the brightest)[13]

"Girasol opal" is a term sometimes mistakenly and improperly used to refer to fire opals, as well as a type of transparent to semitransparent type milky quartz from Madagascar which displays an asterism, or star effect when cut properly. However, the true girasol opal[14] is a type of hyalite 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, too, when it is from Mexico. The two most notable locations of this type of opal are Oregon and Mexico.[15]

A 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 a play of color. Blue opal also comes from Oregon and Idaho in the Owyhee region, as well as from Nevada around the Virgin Valley.[16]

Opal is also formed by diatoms. Diatoms are a form of algae that, when they die, often form layers at the bottoms of lakes, bays, or oceans. Their cell walls are made up of hydrated silicon dioxide which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope. These cell walls or "tests" form the “grains” for the diatomaceous earth. This sedimentary rock is white, opaque, and chalky in texture.[17] Diatomite has multiple industrial uses such as filtering or adsorbing since it has a fine particle size and very porous nature, and gardening to increase water absorption.

History edit

Opal was rare and very valuable in antiquity. In Europe, it was a gem prized by royalty.[18][19] Until the opening of vast deposits in Australia in the 19th century the only known source was Červenica beyond the Roman frontier in Slovakia.[20] Opal is the national gemstone of Australia.[21]

Sources edit

 
Polished opal from Yowah (Yowah Nut[22]), Queensland
 
Boulder opal, Carisbrooke Station near Winton, Queensland

The primary sources of opal are Australia and Ethiopia, but because of inconsistent and widely varying accountings of their respective levels of extraction, it is difficult to accurately state what proportion of the global supply of opal comes from either country. Australian opal has been cited as accounting for 95–97% of the world's supply of precious opal,[23][24] with the state of South Australia accounting for 80% of the world's supply.[25] In 2012, Ethiopian opal production was estimated to be 14,000 kg (31,000 lb) by the United States Geological Survey.[26] USGS data from the same period (2012), reveals Australian opal production to be $41 million.[27] Because of the units of measurement, it is not possible to directly compare Australian and Ethiopian opal production, but these data and others suggest that the traditional percentages given for Australian opal production may be overstated.[28] Yet, the validity of data in the USGS report appears to conflict with that of Laurs et al.[citation needed] and Mesfin,[citation needed] who estimated the 2012 Ethiopian opal output (from Wegeltena) to be only 750 kg (1,650 lb).

Australia edit

The town of 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 August 1956 at the "Eight Mile" opal field in Coober Pedy. It weighs 17,000 carats (3.4 kg; 7.5 lb) and is 11 inches (280 mm) long, with a height of 4+34 in (120 mm) and a width of 4+12 in (110 mm).[29]

The Mintabie Opal Field in South Australia located about 250 km (160 mi) northwest of Coober Pedy has also produced large quantities of crystal opal and 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.

Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, 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), collected from the Griman Creek Formation.[30] 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 in the south.[31] Its largest quantities are found around Jundah and Quilpie in South West Queensland. Australia also has opalized fossil remains, including dinosaur bones in New South Wales[32] and South Australia,[33] and marine creatures in South Australia.[34]

Ethiopia edit

Opal from Ethiopia

It has been reported that Northern African opal was used to make tools as early as 4000 BC.[citation needed] The first published report of gem opal from Ethiopia appeared in 1994, with the discovery of precious opal in the Menz Gishe District, North Shewa Province.[35] The opal, found mostly in the form of nodules, was of volcanic origin and was found predominantly within weathered layers of rhyolite.[36] This Shewa Province opal was mostly dark brown in color and had a tendency to crack. These qualities made it unpopular in the gem trade. In 2008, a new opal deposit was found approximately 180 km north of Shewa Province, near the town of Wegeltena, in Ethiopia's Wollo Province. The Wollo Province opal was different from the previous Ethiopian opal finds in that it more closely resembled the sedimentary opals of Australia and Brazil, with a light background and often vivid play-of-color.[37] Wollo Province opal, more commonly referred to as "Welo" or "Wello" opal, has become the dominant Ethiopian opal in the gem trade.[38]

 
Gem grade Ethiopian Welo precious opal pendant

Virgin Valley, Nevada edit

 
Multicolored rough opal specimen from Virgin Valley, Nevada, US

The Virgin Valley[39] opal fields of 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 in situ within a subsurface horizon or zone of bentonite, which is considered a "lode" deposit. Opals which have weathered out of the in situ 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,[40] Royal Peacock,[41] Bonanza,[42] Opal Queen,[43] and WRT Stonetree/Black Beauty[44] mines. The largest unpolished black opal in the Smithsonian Institution, known as the "Roebling opal",[45] came out of the tunneled portion of the Rainbow Ridge Mine in 1917, and weighs 2,585 carats (517.0 g; 18.24 oz). The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley, weighing 160 carats (32 g; 1.1 oz), known as the "Black Peacock".[46]

Mexico edit

Fire opal is a transparent to translucent opal with warm body colors of yellow to orange to red. Although fire opals don't usually show any play of color, they occasionally 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.[47] Fire opals that do not show a play of color are sometimes referred to as jelly opals. Mexican opals are sometimes cut in their rhyolitic host material if it is hard enough to allow cutting and polishing. This type of Mexican opal is referred to as a Cantera opal. Another type of opal from Mexico, referred to as Mexican water opal, is a colorless opal that exhibits either a bluish or golden internal sheen.[14]

 
Precious Mexican Fire Opal has bright green and yellow patches (play of color) with an orange-red background typical of Fire Opal.

Opal occurs in significant quantity and variety in central Mexico, where mining and production first originated in the state of Querétaro. In this region the opal deposits are located mainly in the mountain ranges of three municipalities: Colón, Tequisquiapan, and Ezequiel Montes. During the 1960s through to the mid-1970s, the Querétaro mines were heavily mined. Today's opal miners report that it was much easier to find quality opals with a lot of fire and play of color back then, whereas today the gem-quality opals are very hard to come by and command hundreds of US dollars or more. The orange-red background color is characteristic of all "fire opals," including "Mexican fire opal".

The oldest mine in Querétaro is Santa Maria del Iris. This mine was opened around 1870 and has been reopened at least 28 times since. At the moment there are about 100 mines in the regions around Querétaro, but most of them are now closed. The best quality of opals came from the mine Santa Maria del Iris, followed by La Hacienda la Esperanza, Fuentezuelas, La Carbonera, and La Trinidad. Important deposits in the state of Jalisco were not discovered until the late 1950s.

In 1957, Alfonso Ramirez (of Querétaro) accidentally discovered the first opal mine in Jalisco: La Unica, located on the outer area of the volcano of Tequila, near the Huitzicilapan farm in Magdalena. By 1960 there were around 500 known opal mines in this region alone. Other regions of the country that also produce opals (of lesser quality) are Guerrero, which produces an opaque opal similar to the opals from Australia (some of these opals are carefully treated with heat to improve their colors so high-quality opals from this area may be suspect). There are also some small opal mines in Morelos, Durango, Chihuahua, Baja California, Guanajuato, Puebla, Michoacán, and Estado de México.

Other locations edit

 
Brazilian opal

Another source of white base opal or creamy opal in the United States is Spencer, Idaho.[48] A high percentage of the opal found there occurs in thin layers.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Canada, Slovakia, Hungary, Turkey, Indonesia, Brazil (in Pedro II, Piauí[49]), Honduras (more precisely in Erandique), Guatemala, and Nicaragua.

In late 2008, NASA announced the discovery of opal deposits on Mars.[50]

Fossil Opal edit

 
Wood opal
 
Precious opal replacing ichthyosaur backbone, as a display specimen in South Australian Museum

Wood opal, also known as xylopal,[51][52] is a form of opal,[53] as well as a type of petrified wood which has developed an opalescent sheen or, more rarely, where the wood has been completely replaced by opal. Other names for this opalized sheen-like wood are opalized wood and opalized petrified wood. It is often used as a gemstone.[54]

Synthetic opal edit

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.[9] 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 fluoresce under ultraviolet light. Synthetics are also generally lower in density and are often highly porous.

Opals which have been created in a laboratory are often termed "lab-created opals", which, while classifiable as man-made and synthetic, are very different from their resin-based counterparts which are also considered man-made and synthetic. The term "synthetic" implies that a stone has been created to be chemically and structurally indistinguishable from a genuine one, and genuine opal contains no resins or polymers. The finest modern lab-created opals do not exhibit the lizard skin or columnar patterning of earlier lab-created varieties, and their patterns are non-directional. They can still be distinguished from genuine opals, however, by their lack of inclusions and the absence of any surrounding non-opal matrix. While many genuine opals are cut and polished without a matrix, the presence of irregularities in their play-of-color continues to mark them as distinct from even the best lab-created synthetics.

Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.[55] Synthetic opals are also deeply investigated in photonics for sensing and light management purposes.[56][57]

Local atomic structure of opals edit

 
The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.
 
Lussatite (opal-CT)

The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, no unit cell 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.[58] 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 SiOH, 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 consists 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 edit

Microcrystalline opal or Opal-CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite over very short length scales. The spheres of opal in microcrystalline opal are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite. Microcrystalline opal has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%.[59] Opal-CT, also called lussatine or lussatite, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5 wt%.

Noncrystalline opal edit

 
Schematic representation of the hydrated opal surface.

Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A" ("A" stands for "amorphous"),[60] 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.

Noncrystalline 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.[61]

Opal surface chemical groups edit

The surface of opal in contact with water is covered by siloxane bonds (≡Si–O–Si≡) and silanol groups (≡Si–OH). This makes the opal surface very hydrophilic and capable of forming numerous hydrogen bonds.

Etymology edit

The word 'opal' is adapted from the Latin term opalus. The origin of this word in turn is a matter of debate, but most modern references suggest it is adapted from the Sanskrit word úpala meaning ‘precious stone’.[62]

References to the gem are made by Pliny the Elder. It is suggested to have been adapted from Ops, the wife of Saturn, and goddess of fertility. The portion of Saturnalia devoted to Ops was "Opalia", similar to opalus.

Another common claim that the term is adapted from the Ancient 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 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.[62]

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.

Historical superstitions edit

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.[63] It was also said to grant invisibility if wrapped in a fresh bay leaf and held in the hand.[63][64] As a result, the opal was seen as the patron gemstone for thieves during the medieval period.[65] Following the publication of Sir Walter Scott's Anne of Geierstein in 1829, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opal 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.[63] 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 20 years or so.[66]

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, as the opal was believed to embody the evil eye.[63]

Opal is considered the birthstone for people born in October.[67]

Examples edit

See also edit

References edit

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  2. ^ a b c d e f g h i j k l m Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  3. ^ "Opal". Webmineral. from the original on 18 October 2011. Retrieved 8 October 2011.
  4. ^ "Opal". Mindat.org. from the original on 6 October 2011. Retrieved 8 October 2011.
  5. ^ "Opal Description". Gemological Institute of America. Retrieved 4 June 2018.
  6. ^ "Glossary: Play of Color". Mindat. Retrieved 4 June 2018.
  7. ^ Sanders, J. V. (1964). "Colour of precious opal". Nature. 204 (496): 1151–1153. Bibcode:1964Natur.204.1151S. doi:10.1038/2041151a0. S2CID 4169953.
  8. ^ Sanders, J. V. (1968). "Diffraction of light by opals". Acta Crystallographica A. 24 (4): 427–434. Bibcode:1968AcCrA..24..427S. doi:10.1107/S0567739468000860.
  9. ^ a b Klein, Cornelis; Hurlbut, Cornelius S. (1985). Manual of Mineralogy (20th ed.). ISBN 978-0-471-80580-9.
  10. ^ Astratov, V. N.; Bogomolov, V. N.; Kaplyanskii, A. A.; Prokofiev, A. V.; Samoilovich, L. A.; Samoilovich, S. M.; Vlasov, Yu. A. (1995). "Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects". Il Nuovo Cimento D. 17 (11–12): 1349–1354. Bibcode:1995NCimD..17.1349A. doi:10.1007/bf02457208. S2CID 121167426.
  11. ^ Dr. Joel Arem; Donald Clark, CSM IMG (23 June 2015). "Opal Value, Price, and Jewelry". Gemsociety.org. from the original on 23 November 2016. Retrieved 22 November 2016.
  12. ^ Gribble, C. D. (1988). "Tektosilicates (framework silicates)". Rutley's Elements of Mineralogy (27th ed.). London: Unwin Hyman. p. 431. ISBN 978-0-04-549011-0.
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  15. ^ Page, Geology (21 June 2016). "Types of Opal". Geology Page. Retrieved 29 December 2022.
  16. ^ "Opal – Nevada Mining Association". www.nevadamining.org. 19 December 2017. Retrieved 18 June 2020.
  17. ^ "diatomite - The Mineral and Gemstone Kingdom". www.minerals.net. Retrieved 15 May 2018.
  18. ^ "The central stone on the thirteenth-century crown of the Holy Roman Emperor was an opal said to be the color of pure white snow, sparkling with splashes of bright red wine: it was called "the Orphanus", perhaps because there was no other stone like it.24 And on New Year's Day 1584,25 Queen Elizabeth I was delighted to receive an opal parure—a full set of matching jewelry—from one of her favorite courtiers, Sir Christopher Hatton." Finlay, Victoria. Jewels: A Secret History (Kindle Locations 2145-2148). Random House Publishing Group. Kindle Edition.
  19. ^ "All of these early stones were almost certainly from the ancient mines of Slovakia—the same source as Nonius’ precious stone and the Holy Roman Emperor’s red-and-white one. The mines were worked until the late nineteenth century..." Finlay, Victoria. Jewels: A Secret History (Kindle Locations 2163-2165). Random House Publishing Group. Kindle Edition.
  20. ^ Finlay, Victoria. Jewels: A Secret History (Kindle Location 1871). Random House Publishing Group. Kindle Edition.
  21. ^ "Australian National Gemstone". Australian Government, Department of the Prime Minister and Cabinet. Retrieved 10 June 2018.
  22. ^ "Yowah Nut: Yowah Nut mineral information and data". Mindat.org. 20 February 2011. from the original on 12 May 2011. Retrieved 8 March 2011.
  23. ^ "Gemstone". It's an Honour. Australian Government. from the original on 29 January 2011.
  24. ^ "Rapaport Magazine – A Designer Stone". from the original on 6 October 2014.
  25. ^ . Government of South Australia. Archived from the original on 16 July 2012. Retrieved 11 July 2012.
  26. ^ Yager, Thomas R. (1 December 2013), "The Mineral Industry of Ethiopia", United States Geological Survey, 2012 Minerals Yearbook (PDF), pp. 17.1–17.5, (PDF) from the original on 6 October 2014
  27. ^ Tse, Pui-Kwan (1 December 2013), "The Mineral Industry of Australia", United States Geological Survey, 2012 Minerals Yearbook (PDF), pp. 3.1–3.27, (PDF) from the original on 6 October 2014, retrieved 20 September 2014
  28. ^ "Rapaport Magazine – Ethiopian Opal". from the original on 6 October 2014.
  29. ^ Cram, Len (2006). A history of South Australian opal, 1840–2005. Lightning Ridge, NSW. ISBN 978-0975721407.{{cite book}}: CS1 maint: location missing publisher (link)
  30. ^ Osborne, Zoe; Kitanov, Alex (17 July 2022). "The black-opal hunters of outback Australia". Al Jazeera. Retrieved 8 August 2022.
  31. ^ . Archived from the original on 13 October 2007.
  32. ^ First dinosaur named in NSW in nearly a century after chance discovery ABC News, 5 December 2018. Retrieved 5 December 2018.
  33. ^ Rare dinosaur fossil discovered on internet after disappearing for decades ABC News, 3 December 2018. Retrieved 3 December 2018.
  34. ^ a b "Opal Fossils". South Australian Museum. from the original on 13 February 2014. Retrieved 3 March 2014.
  35. ^ Barot, N. (1994). "New precious opal deposit found in Ethiopia". ICA Gazette.
  36. ^ Johnson, Mary L.; Kammerling, Robert C.; DeGhionno, Dino G.; Koivula, John I. (Summer 1996). "Opal from Shewa Province, Ethiopia". Gems & Gemology. 32 (2): 112–120. doi:10.5741/GEMS.32.2.112.
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Further reading edit

External links edit

  • Farlang opal Hist. References Localities, anecdotes by Theophrastus, Isaac Newton, Georg Agricola etc.
  • ICA's Opal Page: International Colored Stone Association
  • Opal Fossils from the South Australian Museum 13 February 2014 at the Wayback Machine Accessed 19 October 2016.
  • Opal Mineral data and specimen images Mineralogy Database
  • Opalworld Australian Opal Fields – Map of precious opal deposits

opal, german, subsidiary, brand, stallantis, opel, other, uses, disambiguation, redirects, here, other, uses, disambiguation, hydrated, amorphous, form, silica, sio2, nh2o, water, content, range, from, weight, usually, between, amorphous, property, classified,. For German subsidiary of the car brand Stallantis see Opel For other uses see Opal disambiguation Opals redirects here For other uses see Opals disambiguation Opal is a hydrated amorphous form of silica SiO2 nH2O its water content may range from 3 to 21 by weight but is usually between 6 and 10 Due to its amorphous property it is classified as a mineraloid unlike crystalline forms of silica which are considered minerals It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock being most commonly found with limonite sandstone rhyolite marl and basalt OpalA rich seam of iridescent opal encased in matrixGeneralCategoryMineraloidFormula repeating unit Hydrated silica SiO2 nH2OIMA symbolOpl 1 Crystal systemAmorphous 2 IdentificationColorColorless white yellow red orange green brown black blue pinkCrystal habitIrregular veins in masses in nodulesCleavageNone 2 FractureConchoidal to uneven 2 Mohs scale hardness5 5 6 2 LusterSubvitreous to waxy 2 StreakWhiteDiaphaneityopaque translucent transparentSpecific gravity2 15 0 08 0 90 2 Density2 09 g cm3Polish lusterVitreous to resinous 2 Optical propertiesSingle refractive often anomalous double refractive due to strain 2 Refractive index1 450 0 020 0 080 Mexican opal may read as low as 1 37 but typically reads 1 42 1 43 2 Birefringencenone 2 PleochroismNone 2 Ultraviolet fluorescenceblack 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 2 Absorption spectragreen stones 660 nm 470 nm cutoff 2 Diagnostic featuresdarkening upon heatingSolubilityhot salt water bases methanol humic acid hydrofluoric acidReferences 3 4 Main opal producing countriesThe name opal is believed to be derived from the Sanskrit word upala उपल which means jewel and later the Greek derivative opallios ὀpallios There are two broad classes of opal precious and common Precious opal displays play of color iridescence common opal does not 5 Play of color is defined as a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals as they are turned in white light 6 The internal structure of precious opal causes it to diffract light resulting in play of color Depending on the conditions in which it formed opal may be transparent translucent or opaque and the background color may be white black or nearly any color of the visual spectrum Black opal is considered the rarest while white gray and green opals are the most common Contents 1 Precious opal 2 Common opal 3 Varieties of Common Opal 4 History 5 Sources 5 1 Australia 5 2 Ethiopia 5 3 Virgin Valley Nevada 5 4 Mexico 5 5 Other locations 6 Fossil Opal 7 Synthetic opal 8 Local atomic structure of opals 8 1 Microcrystalline opal 8 2 Noncrystalline opal 8 3 Opal surface chemical groups 9 Etymology 10 Historical superstitions 11 Examples 12 See also 13 References 14 Further reading 15 External linksPrecious opal edit nbsp Precious opal consists of spheres of silica molecules arranged in regular closely packed planes idealized diagram nbsp This precious rough opal from Coober Pedy South Australia displays nearly every color of the visible spectrum Precious opal shows a variable interplay of internal colors and though it is a mineraloid it has an internal structure At microscopic scales precious opal is composed of silica spheres some 150 300 nm 5 9 10 6 1 18 10 5 in in diameter in a hexagonal or cubic close packed lattice It was shown by J V Sanders in the mid 1960s 7 8 that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal 9 The regularity of the sizes and the packing of these spheres is a prime determinant of the quality of precious opal Where the distance between the regularly packed planes of spheres is around 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 colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light The process can be described by Bragg s law of diffraction Visible light cannot pass through large thicknesses of the opal This is the basis of the optical band gap in a photonic crystal The notion that opals are photonic crystals for visible light was expressed in 1995 by Vasily Astratov s group 10 In addition microfractures may be filled with secondary silica and form thin lamellae inside the opal during its formation The term opalescence is commonly used to describe this unique and beautiful phenomenon which in gemology is termed play of color In gemology opalescence is applied to the hazy milky turbid sheen of common or potch opal which does not show a play of color clarification needed Opalescence is a form of adularescence For gemstone use most opal is cut and polished to form a cabochon Natural opal refers to polished stones consisting wholly of precious opal Opals too thin to produce a natural opal may be combined with other materials to form composite 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 an inferior specimen or an extremely thin section of precious opal Triplet opals tend to have a more artificial appearance and are not classed as precious gemstones but rather composite gemstones Jewelry applications of precious opal can be somewhat limited by opal s sensitivity to heat due primarily to its relatively high water content and predisposition to scratching 11 Combined with modern techniques of polishing a doublet opal can produce a similar effect to Natural black or boulder opal at a 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 Common opal edit nbsp Common rough opal nbsp White and blue opal from SlovakiaBesides the gemstone varieties that show a play of color the other kinds of common opal include 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 12 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 geysers and diatomaceous earth the accumulations of diatom shells or tests Common opal often displays a hazy milky turbid sheen from within the stone In gemology this optical effect is strictly defined as opalescence which is a form of adularescence Varieties of Common Opal edit Girasol redirects here For the American sunflower see Jerusalem artichoke nbsp Brazilian Precious Opal with patches of brilliant color throughout The brightness of the fire in opal ranges on a scale of 1 to 5 with 5 being the brightest 13 Girasol opal is a term sometimes mistakenly and improperly used to refer to fire opals as well as a type of transparent to semitransparent type milky quartz from Madagascar which displays an asterism or star effect when cut properly However the true girasol opal 14 is a type of hyalite 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 too when it is from Mexico The two most notable locations of this type of opal are Oregon and Mexico 15 A 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 a play of color Blue opal also comes from Oregon and Idaho in the Owyhee region as well as from Nevada around the Virgin Valley 16 Opal is also formed by diatoms Diatoms are a form of algae that when they die often form layers at the bottoms of lakes bays or oceans Their cell walls are made up of hydrated silicon dioxide which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope These cell walls or tests form the grains for the diatomaceous earth This sedimentary rock is white opaque and chalky in texture 17 Diatomite has multiple industrial uses such as filtering or adsorbing since it has a fine particle size and very porous nature and gardening to increase water absorption History editOpal was rare and very valuable in antiquity In Europe it was a gem prized by royalty 18 19 Until the opening of vast deposits in Australia in the 19th century the only known source was Cervenica beyond the Roman frontier in Slovakia 20 Opal is the national gemstone of Australia 21 Sources edit nbsp Polished opal from Yowah Yowah Nut 22 Queensland nbsp Boulder opal Carisbrooke Station near Winton QueenslandThe primary sources of opal are Australia and Ethiopia but because of inconsistent and widely varying accountings of their respective levels of extraction it is difficult to accurately state what proportion of the global supply of opal comes from either country Australian opal has been cited as accounting for 95 97 of the world s supply of precious opal 23 24 with the state of South Australia accounting for 80 of the world s supply 25 In 2012 Ethiopian opal production was estimated to be 14 000 kg 31 000 lb by the United States Geological Survey 26 USGS data from the same period 2012 reveals Australian opal production to be 41 million 27 Because of the units of measurement it is not possible to directly compare Australian and Ethiopian opal production but these data and others suggest that the traditional percentages given for Australian opal production may be overstated 28 Yet the validity of data in the USGS report appears to conflict with that of Laurs et al citation needed and Mesfin citation needed who estimated the 2012 Ethiopian opal output from Wegeltena to be only 750 kg 1 650 lb Australia edit The town of 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 August 1956 at the Eight Mile opal field in Coober Pedy It weighs 17 000 carats 3 4 kg 7 5 lb and is 11 inches 280 mm long with a height of 4 3 4 in 120 mm and a width of 4 1 2 in 110 mm 29 The Mintabie Opal Field in South Australia located about 250 km 160 mi northwest of Coober Pedy has also produced large quantities of crystal opal and 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 Andamooka in South Australia is also a major producer of matrix opal crystal opal and black opal Another Australian town 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 collected from the Griman Creek Formation 30 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 in the south 31 Its largest quantities are found around Jundah and Quilpie in South West Queensland Australia also has opalized fossil remains including dinosaur bones in New South Wales 32 and South Australia 33 and marine creatures in South Australia 34 Ethiopia edit source source source source source source source Opal from EthiopiaIt has been reported that Northern African opal was used to make tools as early as 4000 BC citation needed The first published report of gem opal from Ethiopia appeared in 1994 with the discovery of precious opal in the Menz Gishe District North Shewa Province 35 The opal found mostly in the form of nodules was of volcanic origin and was found predominantly within weathered layers of rhyolite 36 This Shewa Province opal was mostly dark brown in color and had a tendency to crack These qualities made it unpopular in the gem trade In 2008 a new opal deposit was found approximately 180 km north of Shewa Province near the town of Wegeltena in Ethiopia s Wollo Province The Wollo Province opal was different from the previous Ethiopian opal finds in that it more closely resembled the sedimentary opals of Australia and Brazil with a light background and often vivid play of color 37 Wollo Province opal more commonly referred to as Welo or Wello opal has become the dominant Ethiopian opal in the gem trade 38 nbsp Gem grade Ethiopian Welo precious opal pendantVirgin Valley Nevada edit nbsp Multicolored rough opal specimen from Virgin Valley Nevada USThe Virgin Valley 39 opal fields of 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 in situ within a subsurface horizon or zone of bentonite which is considered a lode deposit Opals which have weathered out of the in situ 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 40 Royal Peacock 41 Bonanza 42 Opal Queen 43 and WRT Stonetree Black Beauty 44 mines The largest unpolished black opal in the Smithsonian Institution known as the Roebling opal 45 came out of the tunneled portion of the Rainbow Ridge Mine in 1917 and weighs 2 585 carats 517 0 g 18 24 oz The largest polished black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley weighing 160 carats 32 g 1 1 oz known as the Black Peacock 46 Mexico editFire opal is a transparent to translucent opal with warm body colors of yellow to orange to red Although fire opals don t usually show any play of color they occasionally exhibit bright green flashes The most famous source of fire opals is the state of Queretaro in Mexico these opals are commonly called Mexican fire opals 47 Fire opals that do not show a play of color are sometimes referred to as jelly opals Mexican opals are sometimes cut in their rhyolitic host material if it is hard enough to allow cutting and polishing This type of Mexican opal is referred to as a Cantera opal Another type of opal from Mexico referred to as Mexican water opal is a colorless opal that exhibits either a bluish or golden internal sheen 14 nbsp Precious Mexican Fire Opal has bright green and yellow patches play of color with an orange red background typical of Fire Opal Opal occurs in significant quantity and variety in central Mexico where mining and production first originated in the state of Queretaro In this region the opal deposits are located mainly in the mountain ranges of three municipalities Colon Tequisquiapan and Ezequiel Montes During the 1960s through to the mid 1970s the Queretaro mines were heavily mined Today s opal miners report that it was much easier to find quality opals with a lot of fire and play of color back then whereas today the gem quality opals are very hard to come by and command hundreds of US dollars or more The orange red background color is characteristic of all fire opals including Mexican fire opal The oldest mine in Queretaro is Santa Maria del Iris This mine was opened around 1870 and has been reopened at least 28 times since At the moment there are about 100 mines in the regions around Queretaro but most of them are now closed The best quality of opals came from the mine Santa Maria del Iris followed by La Hacienda la Esperanza Fuentezuelas La Carbonera and La Trinidad Important deposits in the state of Jalisco were not discovered until the late 1950s In 1957 Alfonso Ramirez of Queretaro accidentally discovered the first opal mine in Jalisco La Unica located on the outer area of the volcano of Tequila near the Huitzicilapan farm in Magdalena By 1960 there were around 500 known opal mines in this region alone Other regions of the country that also produce opals of lesser quality are Guerrero which produces an opaque opal similar to the opals from Australia some of these opals are carefully treated with heat to improve their colors so high quality opals from this area may be suspect There are also some small opal mines in Morelos Durango Chihuahua Baja California Guanajuato Puebla Michoacan and Estado de Mexico Other locations edit nbsp Brazilian opalAnother source of white base opal or creamy opal in the United States is Spencer Idaho 48 A high percentage of the opal found there occurs in thin layers Other significant deposits of precious opal around the world can be found in the Czech Republic Canada Slovakia Hungary Turkey Indonesia Brazil in Pedro II Piaui 49 Honduras more precisely in Erandique Guatemala and Nicaragua In late 2008 NASA announced the discovery of opal deposits on Mars 50 Fossil Opal edit nbsp Wood opal nbsp Precious opal replacing ichthyosaur backbone as a display specimen in South Australian MuseumWood opal also known as xylopal 51 52 is a form of opal 53 as well as a type of petrified wood which has developed an opalescent sheen or more rarely where the wood has been completely replaced by opal Other names for this opalized sheen like wood are opalized wood and opalized petrified wood It is often used as a gemstone 54 Synthetic opal editOpals 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 9 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 fluoresce under ultraviolet light Synthetics are also generally lower in density and are often highly porous Opals which have been created in a laboratory are often termed lab created opals which while classifiable as man made and synthetic are very different from their resin based counterparts which are also considered man made and synthetic The term synthetic implies that a stone has been created to be chemically and structurally indistinguishable from a genuine one and genuine opal contains no resins or polymers The finest modern lab created opals do not exhibit the lizard skin or columnar patterning of earlier lab created varieties and their patterns are non directional They can still be distinguished from genuine opals however by their lack of inclusions and the absence of any surrounding non opal matrix While many genuine opals are cut and polished without a matrix the presence of irregularities in their play of color continues to mark them as distinct from even the best lab created synthetics Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics 55 Synthetic opals are also deeply investigated in photonics for sensing and light management purposes 56 57 Local atomic structure of opals edit nbsp The crystal structure of crystalline a cristobalite Locally the structures of some opals opal C are similar to this nbsp Lussatite opal CT The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica As a mineraloid no unit cell 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 58 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 SiOH 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 consists 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 edit Microcrystalline opal or Opal CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite over very short length scales The spheres of opal in microcrystalline opal are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite Microcrystalline opal has occasionally been further subdivided in the literature Water content may be as high as 10 wt 59 Opal CT also called lussatine or lussatite is interpreted as consisting of localized order of a cristobalite with a lot of stacking disorder Typical water content is about 1 5 wt Noncrystalline opal edit nbsp Schematic representation of the hydrated opal surface Two broad categories of noncrystalline opals sometimes just referred to as opal A A stands for amorphous 60 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 Noncrystalline 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 61 Opal surface chemical groups edit The surface of opal in contact with water is covered by siloxane bonds Si O Si and silanol groups Si OH This makes the opal surface very hydrophilic and capable of forming numerous hydrogen bonds Etymology editThe word opal is adapted from the Latin term opalus The origin of this word in turn is a matter of debate but most modern references suggest it is adapted from the Sanskrit word upala meaning precious stone 62 References to the gem are made by Pliny the Elder It is suggested to have been adapted from Ops the wife of Saturn and goddess of fertility The portion of Saturnalia devoted to Ops was Opalia similar to opalus Another common claim that the term is adapted from the Ancient 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 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 62 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 Historical superstitions editIn 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 63 It was also said to grant invisibility if wrapped in a fresh bay leaf and held in the hand 63 64 As a result the opal was seen as the patron gemstone for thieves during the medieval period 65 Following the publication of Sir Walter Scott s Anne of Geierstein in 1829 opal acquired a less auspicious reputation In Scott s novel the Baroness of Arnheim wears an opal 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 63 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 20 years or so 66 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 as the opal was believed to embody the evil eye 63 Opal is considered the birthstone for people born in October 67 Examples editThe Olympic Australis the world s largest and most valuable gem opal found in Coober Pedy 68 69 The Andamooka Opal presented to Queen Elizabeth II also known as the Queen s Opal The Addyman Plesiosaur from Andamooka the finest known opalised skeleton on Earth 34 The Burning of Troy the now lost opal presented to Josephine de Beauharnais by Napoleon I of France and the first named opal 70 The Flame Queen Opal The Halley s Comet Opal the world s largest uncut black opal Although the clock faces above the information stand in Grand Central Terminal in New York City are often said to be opal they are in fact opalescent glass The Roebling Opal Smithsonian Institution 71 The Galaxy Opal listed as the World s Largest Polished Opal in the 1992 Guinness Book of Records 72 The Rainbow Virgin the finest crystal opal specimen ever unearthed 73 74 75 The Sea of Opal the largest black opal in the world 76 The Fire of Australia assumed to be the finest uncut opal in existence 75 77 Beverly the Bug the first known example of an opal with an insect inclusionSee also edit nbsp Minerals portalBiogenic silica Cacholong Foil opal Labradorite Opalite Optical phenomena Observable events that result from the interaction of light and matterPages displaying short descriptions of redirect targets Uncut Gems 2019 film References edit Warr L N 2021 IMA CNMNC approved mineral symbols Mineralogical Magazine 85 3 291 320 Bibcode 2021MinM 85 291W doi 10 1180 mgm 2021 43 S2CID 235729616 a b c d e f g h i j k l m Gemological Institute of America GIA Gem Reference Guide 1995 ISBN 0 87311 019 6 Opal Webmineral Archived from the original on 18 October 2011 Retrieved 8 October 2011 Opal Mindat org Archived from the original on 6 October 2011 Retrieved 8 October 2011 Opal Description Gemological Institute of America Retrieved 4 June 2018 Glossary Play of Color Mindat Retrieved 4 June 2018 Sanders J V 1964 Colour of precious opal Nature 204 496 1151 1153 Bibcode 1964Natur 204 1151S doi 10 1038 2041151a0 S2CID 4169953 Sanders J V 1968 Diffraction of light by opals Acta Crystallographica A 24 4 427 434 Bibcode 1968AcCrA 24 427S doi 10 1107 S0567739468000860 a b Klein Cornelis Hurlbut Cornelius S 1985 Manual of Mineralogy 20th ed ISBN 978 0 471 80580 9 Astratov V N Bogomolov V N Kaplyanskii A A Prokofiev A V Samoilovich L A Samoilovich S M Vlasov Yu A 1995 Optical spectroscopy of opal matrices with CdS embedded in its pores Quantum confinement and photonic band gap effects Il Nuovo Cimento D 17 11 12 1349 1354 Bibcode 1995NCimD 17 1349A doi 10 1007 bf02457208 S2CID 121167426 Dr Joel Arem Donald Clark CSM IMG 23 June 2015 Opal Value Price and Jewelry Gemsociety org Archived from the original on 23 November 2016 Retrieved 22 November 2016 Gribble C D 1988 Tektosilicates framework silicates Rutley s Elements of Mineralogy 27th ed London Unwin Hyman p 431 ISBN 978 0 04 549011 0 Downing Paul B 1992 Opal Identification and Value pp 55 61 a b Swisher James Anthony Edna B Let s Talk Gemstones Opal Archived from the original on 7 November 2011 Page Geology 21 June 2016 Types of Opal Geology Page Retrieved 29 December 2022 Opal Nevada Mining Association www nevadamining org 19 December 2017 Retrieved 18 June 2020 diatomite The Mineral and Gemstone Kingdom www minerals net Retrieved 15 May 2018 The central stone on the thirteenth century crown of the Holy Roman Emperor was an opal said to be the color of pure white snow sparkling with splashes of bright red wine it was called the Orphanus perhaps because there was no other stone like it 24 And on New Year s Day 1584 25 Queen Elizabeth I was delighted to receive an opal parure a full set of matching jewelry from one of her favorite courtiers Sir Christopher Hatton Finlay Victoria Jewels A Secret History Kindle Locations 2145 2148 Random House Publishing Group Kindle Edition All of these early stones were almost certainly from the ancient mines of Slovakia the same source as Nonius precious stone and the Holy Roman Emperor s red and white one The mines were worked until the late nineteenth century Finlay Victoria Jewels A Secret History Kindle Locations 2163 2165 Random House Publishing Group Kindle Edition Finlay Victoria Jewels A Secret History Kindle Location 1871 Random House Publishing Group Kindle Edition Australian National Gemstone Australian Government Department of the Prime Minister and Cabinet Retrieved 10 June 2018 Yowah Nut Yowah Nut mineral information and data Mindat org 20 February 2011 Archived from the original on 12 May 2011 Retrieved 8 March 2011 Gemstone It s an Honour Australian Government Archived from the original on 29 January 2011 Rapaport Magazine A Designer Stone Archived from the original on 6 October 2014 Opal South Australia s Gemstone Government of South Australia Archived from the original on 16 July 2012 Retrieved 11 July 2012 Yager Thomas R 1 December 2013 The Mineral Industry of Ethiopia United States Geological Survey 2012 Minerals Yearbook PDF pp 17 1 17 5 archived PDF from the original on 6 October 2014 Tse Pui Kwan 1 December 2013 The Mineral Industry of Australia United States Geological Survey 2012 Minerals Yearbook PDF pp 3 1 3 27 archived PDF from the original on 6 October 2014 retrieved 20 September 2014 Rapaport Magazine Ethiopian Opal Archived from the original on 6 October 2014 Cram Len 2006 A history of South Australian opal 1840 2005 Lightning Ridge NSW ISBN 978 0975721407 a href Template Cite book html title Template Cite book cite book a CS1 maint location missing publisher link Osborne Zoe Kitanov Alex 17 July 2022 The black opal hunters of outback Australia Al Jazeera Retrieved 8 August 2022 Queensland opal Archived from the original on 13 October 2007 First dinosaur named in NSW in nearly a century after chance discovery ABC News 5 December 2018 Retrieved 5 December 2018 Rare dinosaur fossil discovered on internet after disappearing for decades ABC News 3 December 2018 Retrieved 3 December 2018 a b Opal Fossils South Australian Museum Archived from the original on 13 February 2014 Retrieved 3 March 2014 Barot N 1994 New precious opal deposit found in Ethiopia ICA Gazette Johnson Mary L Kammerling Robert C DeGhionno Dino G Koivula John I Summer 1996 Opal from Shewa Province Ethiopia Gems amp Gemology 32 2 112 120 doi 10 5741 GEMS 32 2 112 Rondeau Benjamin Summer 2010 Play of color from Wegeltena Wollo Province Ethiopia Gems amp Gemology doi 10 5741 GEMS 46 2 90 Gashaw Yidneka 8 April 2012 Opal Trade Transforms North Wollo Addis Fortune Archived from the original on 24 September 2014 Virgin Valley District Humboldt Co Nevada mindat org Archived from the original on 30 April 2012 Rainbow Ridge Mine mindat org Archived from the original on 30 April 2012 Royal Peacock Group Mines Virgin Valley District Humboldt Co Nevada mindat org Archived from the original on 30 April 2012 Bonanza Opal Workings Virgin Opal Virgin Valley District Humboldt Co Nevada mindat org Archived from the original on 30 April 2012 Opal Queen group Virgin Valley District Humboldt Co Nevada mindat org Archived from the original on 30 April 2012 Stonetree Opal Mine WRT Stonetree group Virgin Valley District Humboldt Co Nevada mindat org Archived from the original on 30 April 2012 Roebling Opal National Museum of Natural History Archived from the original on 7 November 2012 Opal Lapidary Journal 1522 1542 March 1971 Koivula John 1983 OPAL FROM QUERETARO MEXICO OCCURRENCE AND INCLUSIONS Gems amp Gemology 19 2 87 96 doi 10 5741 GEMS 19 2 87 If you ve ever wanted an opal it s time to visit Spencer East Idaho News East Idaho News 6 August 2015 Retrieved 14 February 2018 Boi Morto Mine Pedro II Piaui Brazil Mindat org Archived from the original on 12 May 2011 Retrieved 8 October 2011 NASA probe finds opals in Martian crevices Jet Propulsion Laboratory Archived from the original on 19 January 2009 Retrieved 29 October 2008 Xylopal mindat org Retrieved 6 June 2023 Wood Opal mindat org mindat org Retrieved 6 June 2023 Egleston T 1887 Catalogue of minerals and synonyms Washington Gov Print Off p 238 OCLC 918261679 Retrieved 10 July 2023 Opalised Wood geology com Retrieved 6 June 2023 Macroporous Structures Metal Oxides Highly Ordered Office for Technology Commercialization Technology Marketing Site University of Minnesota 25 June 2010 Archived from the original on 24 March 2012 Retrieved 8 October 2011 Lova Paola Congiu Simone Sparnacci Katia Angelini Angelo Boarino Luca Laus Michele Stasio Francesco Di Comoretto Davide 8 April 2020 Core shell silica rhodamine B nanosphere for synthetic opals from fluorescence spectral redistribution to sensing RSC Advances 10 25 14958 14964 Bibcode 2020RSCAd 1014958L doi 10 1039 D0RA02245D ISSN 2046 2069 PMC 9052040 PMID 35497145 Tetreault N Miguez H Ozin G A 2004 Silicon Inverse Opal A Platform for Photonic Bandgap Research Advanced Materials 16 16 1471 1476 Bibcode 2004AdM 16 1471T doi 10 1002 adma 200400618 ISSN 1521 4095 S2CID 137194866 Graetsch H 1994 Heaney P J Prewitt Connecticut Gibbs G V eds Structural characteristics of opaline and microcrystalline silica minerals Silica physical behavior geochemistry and materials applications Reviews in Mineralogy 29 Opal CT on Midat Wilson M J 2014 The structure of opal CT revisited Journal of Non Crystalline Solids 405 68 Bibcode 2014JNCS 405 68W doi 10 1016 j jnoncrysol 2014 08 052 Retrieved 16 June 2022 Cady S L Wenk H R Downing K H 1996 HRTEM of microcrystalline opal in chert and porcelanite from the Monterey Formation California PDF American Mineralogist 81 11 12 1380 1395 Bibcode 1996AmMin 81 1380C doi 10 2138 am 1996 11 1211 S2CID 53527412 Archived PDF from the original on 1 October 2011 a b Eckert Allan W 1997 The World of Opals John Wiley and Sons pp 56 57 ISBN 9780471133971 Archived from the original on 26 April 2016 a b c d Fernie William Thomas 1907 Precious Stones for Curative Wear Bristol John Wright amp Co pp 248 249 Dunwich Gerina 1996 Wicca Candle Magick pp 84 85 Opal Symbolism and Legends International Gem Society Retrieved 16 October 2022 Eckert Allan W 1997 A Chronological History and Mythology of Opals The World of Opals New York John Wiley amp Sons pp 53 118 Goldberg Gist Arlene 2003 What s that Stuff Opal Chemical amp Engineering News 81 4 doi 10 1021 cen v081n004 p058 Leechman F The opal book University of California Press 1961 opalsdownunder com au Eckert Allan W 1997 The World of Opals Chichester John Wiley amp Sons pp 67 126 ISBN 978 0 471 13397 1 The Dynamic Earth National Museum of Natural History Archived from the original on 7 June 2011 Retrieved 8 October 2011 Guinness Medd October 1992 The Guinness Book of Records 1993 Guinness p 22 ISBN 978 0 85112 978 5 Archived from the original on 26 August 2016 Million dollar opal Rainbow Virgin to go on display at SA exhibition celebrating centenary ABC News 2 August 2015 Archived from the original on 17 December 2017 Retrieved 13 January 2016 Opals media releases South Australian Museum Archived from the original on 29 December 2015 Retrieved 13 January 2016 a b 4 million worth of the South Australian Museum s opal collection for display in Qatar South Australian Museum Archived from the original on 17 December 2017 Retrieved 7 July 2017 Largest black opal in the world 25 October 2016 Archived from the original on 19 March 2017 Retrieved 18 March 2017 World s finest piece of uncut opal finds new home at the South Australian Museum South Australian Museum Archived from the original on 26 June 2017 Retrieved 7 July 2017 Further reading editEckert Alan 1997 The World of Opals Wiley ISBN 9780471133971 OCLC 36352362 External links edit nbsp Wikimedia Commons has media related to Opal nbsp Look up opal in Wiktionary the free dictionary nbsp Wikisource has original text related to this article 1911 Encyclopaedia Britannica Opal Farlang opal Hist References Localities anecdotes by Theophrastus Isaac Newton Georg Agricola etc ICA s Opal Page International Colored Stone Association Opal Fossils from the South Australian Museum Archived 13 February 2014 at the Wayback Machine Accessed 19 October 2016 Opal Mineral data and specimen images Mineralogy Database Opalworld Australian Opal Fields Map of precious opal deposits Retrieved from https en wikipedia org w index php title Opal amp oldid 1184931650, wikipedia, wiki, book, books, library,

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