Pictures are taken in direct sunlight and are enlarged. To see the play of colors opals must be viewed under a direct light source.
Die Bilder wurden in direktem sonnenlicht aufgenommen und vergrößert. Um das Farbenspiel sehen zu können muß sich der stein unter direkter Lichteinwirkung befinden
A new opal deposit was discovered in 2008 near the village of Wegel Tena, in volcanic rocks of Ethiopia’s Wollo Province. Unlike previous Ethiopian opals, the new material is mostly white, with some brown opal, fire opal, and colorless “crystal” opal. Some of it resembles Australian and Brazilian sedimentary opals, with play-of-color that is often very vivid. However, its properties are consistent with those of opal-CT and most volcanic opals. Inclusions consist of pyrite, bariummanganese oxides, and native carbon. Some samples show “digit patterns”: interpenetrating playof-color and common opal, resembling fingers. The opaque-to-translucent Wegel Tena opals become transparent when soaked in water, showing a remarkable hydrophane character. White opals from this deposit contain an elevated Ba content, which has not been reported so far in opal-CT. The fire and crystal opals are prone to breakage, while the white, opaque-to-translucent opals are remarkably durable. The proportion of gem-quality material in the Wegel Tena deposit seems unusually high, and 1,500 kg have already been extracted using rudimentary mining techniques. The deposit may extend over several kilometers and could become a major source of gem-quality opal.
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The Ethiopian Welo Opal was only discovered in 2008. It differs from the Australian Opal (AO) in many ways. It is found in nodules, where as Australian Opal is found in very thin layers, though both are found in seams as opposed to pockets or scattered in strata. The most sought after and highest quality of AO is "black". So far there have been no confirmed black opals from Ethiopia, but the quality of the fire of Welo opal matches if not exceeds the best AO has to offer. There have been reports of faked black opal from Ethiopia that have been either heat treated or dyed.
Recently the Ethiopian Government made a deal to send the rough Opals to India to be cut, and has dampened the import to the west of rough stones. The government felt it was losing too much money by selling the cheaper rough stones. They had wanted to completely stop export of rough stones, and create an industry to cut and polish the stone for export and increase their profit. Apparently they were unable to do this, and instead made a deal with India.
Some natural opals, mostly from Ethiopia, show a macroscopic finger-like structure called a digit pattern. This pattern consists of vertical columns that are more or less parallel, separated by a homogeneous matrix of different color, transparency, or play-of-color. This study proposes that digits develop through: (1) the deposition of a homogeneous opal layer and subsequent polygonization in the form of vertical columns; (2) preferential alteration of this layer at the vertical grain and sub-grain boundaries, creating the digit shape; (3) precipitation of a new silica gel in the space between the digits; and (4) the drying and solidification of the opal. Although polygonization in the form of vertical columns is a growth process typical of synthetic opal, the post-growth alteration of these columns into digits and the deposition of matrix are observed only in natural opal.Opal is a poorly crystallized or amorphous hydrated silica formed through the solidification of a silica gel (Jones and Segnit, 1971). The most valuable variety is “precious opal,” which displays play-of-color: patches of pure spectral colors from violet to red flashing over the stone as it is tilted. Precious opals are mined in many parts of the world, most notably in Australia, Brazil, Mexico, and Ethiopia. Other sources include the United States, Honduras, and Java. Ethiopia has been a major producer since the 2008 discovery of abundant opal at Wegel Tena, in the northeastern Wollo Province (Rondeau et al., 2009, 2010; Mazzero et al., 2009, 2010).
Play-of-color arises from the diffraction of visible light on monosized, well-ordered silica spheres in opal-A (Sanders 1964; Darragh and Sanders, 1965), and lepispheres in opal-CT (Flörke et al., 1976) of appropriate diameter. Most often, play-of-color involves several juxtaposed patches of various diffraction colors. In rare cases, a network of silica spheres is distributed over the whole stone, so that the color patches move in unison. Such samples are considered natural “photonic” crystals. Whereas a crystal sensu stricto diffracts X-rays, a photonic crystal diffracts wavelengths in the visible range of the spectrum, giving rise to visible play-of-color. The diffraction colors in precious opal can be arranged in a variety of patterns, including intense specks (pinfire), flames, bands, and juxtaposed polygons (harlequin opal). Straight black lines or bands often cross the patches, and these are due to polysynthetic or mechanical twin planes of the photonic crystal, merging at the surface of the gem (Kinder, 1969; Gauthier, 1985). Unlike precious opal, common opals do not display play-of-color, usually because the silica spheres lack regular packing (Gaillou et al., 2008).
Although some rare opals are colorless, most specimens present a bodycolor: white, black, gray, brown, yellow to orange (as in fire opal), red, pink, blue, green, or violet. These colors are typically due to minute mineral inclusions colored by transition metal ions that absorb part of the visible spectrum of light. These include iron for yellow to orange to brown (fire opal), copper for a saturated blue (“Peruvian opal”), and nickel for green (chrysopal; Fritsch et al., 1999). Other causes of color in the inclusions are color centers (purple fluorite inclusions) and organic compounds (quinones in pink opals; Mathey and Luckins, 1998; Fritsch et al., 2004).
In this article, we document an optical feature encountered in some gem opals: Viewed in one direction, the surface shows a mosaic of polygonal to rounded patches of opal, separated by a homogeneous matrix of distinctly different opal. When viewed from the perpendicular direction, these patches appear more elongated and parallel, like columns, that are rounded at one end (figure 1). Each column represents a grain made up of a homogeneous network of silica spheres. The resulting three-dimensional feature’s resemblance to fingers (at the millimeter scale), inspired us to call them digits (Gauthier et al., 2004; Rondeau et al., 2010). This feature has also been described by other gemologists (Hainschwang, 2006; Choudhary, 2008). Digits are most spectacular when the rounded columns possess play-of-color and the matrix is common opal, as shown in figure 1.
Digits are so frequently observed in Ethiopian opals, either from Wegel Tena in Wollo Province (Rondeau et al., 2010) or from Mezezo in Shewa Province (Johnson et al., 1996; Mazzero, 2003) that they have become the industry’s unofficial identifier for Ethiopian opal. Examples have occasionally been reported from other deposits, such as Virgin Valley, Nevada (Gübelin and Koivula, 2005; Gaber, 2007). Digits have been reported in only one Australian sample (figure 2). This paper aims to provide further documentation on digit patterns by proposing a model for their formation.
All Ethiopian samples were rough or cabochon opals collected in the gem market at Addis Ababa, Ethiopia, between 2008 and 2011. Their digit patterns were observable with the unaided eye at the millimeter or centimeter scale (again, see figure 1). From thousands of samples examined, more than 10% of them showed digit patterns. We documented those that showed spectacular or interesting features before they were sold. The specimen from Honduras was photographed during the 2011 Sainte-Marie-aux-Mines gem and mineral show (see figure 14). The specimen from Australia (figure 2) was photographed at the Coober Pedy mine.
We photographed the specimens using a camera equipped with either a macro objective lens or a binocular microscope with up to 50× magnification.
As defined earlier, digits are characterized by their shape and revealed by the optical contrast between the columns and the matrix. This section provides observations and interpretation useful for establishing a formation model.
Variation in the Appearance of the Digits. In samples that contain abundant matrix and few play-of-color patches, the digits are clearly rounded at the ends (again, see figure 1). In samples that contain less matrix and abundant patches, the digit ends are less rounded and more polygonal (figure 3). Some samples display juxtaposed, polygonal columns with no matrix at all in between. In this last case, the digit pattern is no longer visible, and such stones show, in a transversal section, juxtaposed polygonal columns of play-of-color opal (as in the lower right part of figure 3). This is somewhat similar to the harlequin opal.
When digits are polygonal or nearly polygonal (with no matrix or little matrix, respectively), one can observe their cross-sections; these are usually pentagonal or hexagonal. That is, each cross-section is generally five- or six-sided and neighbored by five or six others (again, see figure 3). In only one sample did we encounter polygonal digits with matrix in between with irregular, angular shapes instead of rounded ones (figure 4).
Digits are most pronounced in play-of-color opal, though they also exist very rarely in common opal (again, see figure 2). These digits can be displayed in opals with various bodycolors—from white to gray to brown—and a range of transparencies. The matrix typically has a similar hue but is less transparent, though this is not always the case: Figure 5 shows a sample with colorless to whitish digits embedded in a brownish orange common opal matrix.
Play-of-Color Network Preceding the Formation of Digits. When digits are composed of precious opal, the play-of-color network is usually continuous from one patch to another. In some cases, the whole sample consists of a single network of diffracting opal, subdivided into digits (figure 6). We also observed that twin planes are continuous from one digit to its neighbors (figure 7). These observations indicate that some digits may result from the cross-section partitioning of a larger, preexisting opal. Digit patterns that are not continuous, but display very similar diffraction colors from one patch to the next, may be the result of slight misorientation in the silica sphere network (figure 8).
31ct Crystal Opal Pendant set in 14k gold with Green Diamond Accents
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|
General | |
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Category | Mineraloid |
Formula (repeating unit) |
Hydrated silica. SiO2·nH2O |
Identification | |
Color | Colorless, white, yellow, red, orange, green, brown, black, blue |
Crystal habit | Irregular veins, in masses, in nodules |
Crystal system | Amorphous[1] |
Cleavage | None[1] |
Fracture | Conchoidal to uneven[1] |
Mohs scale hardness | 5.5–6[1] |
Luster | Subvitreous to waxy[1] |
Streak | White |
Diaphaneity | opaque, translucent, transparent |
Specific gravity | 2.15 (+.08, -.90)[1] |
Density | 2.09 |
Polish luster | Vitreous to resinous[1] |
Optical properties | Single refractive, often anomalous double refractive due to strain[1] |
Refractive index | 1.450 (+.020, -.080) Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[1] |
Birefringence | none[1] |
Pleochroism | None[1] |
Ultraviolet 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.[1] |
Absorption spectra | green stones: 660nm, 470nm cutoff[1] |
Diagnostic features | darkening upon heating |
Solubility | hot saltwater, bases, methanol, humic acid, hydrofluoric acid |
References | [2][3] |
Opal is a hydrated amorphous form of 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 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 is the national gemstone of Australia, which produces 97% of the world's supply.[4] This includes the production of the state of South Australia, which amounts to around 80% of the world's supply.[5]
The internal structure of precious opal makes it 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. For gemstone use, its natural color is often enhanced by placing thin layers of opal on a darker underlying stone, like basalt. Common opal, called "potch" by miners, does not show the display of color exhibited in precious opal.[6]
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 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[9] 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 grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.
Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, 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 unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian or black potch. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.
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.
The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear quartz or plastic on top, which 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.
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;[10] 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 diatomite or diatomaceous earth, the accumulations of diatom shells or tests.
Fire opals are transparent to translucent opals with warm body colors of yellow, orange, orange-yellow or red. They do 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.[11]
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[11] 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]
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]
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]
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 (3450 grams) and is 11 inches (280 mm) long, with a height of 4 3⁄4 inches (120 mm) and a width of 4 1⁄2 inches (110 mm).[citation needed]
The 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.
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). 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.[13] Its largest quantities are found around Jundah and Quilpie (known as the "home of the Boulder Opal"[14]) 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] 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]
The Virgin Valley[15] 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 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,[16] Royal Peacock,[17] Bonanza,[18] Opal Queen,[19] and WRT Stonetree/Black Beauty[20] Mines. The largest unpolished Black Opal in the Smithsonian Institution, known as the "Roebling Opal,"[21] 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]
Another source of white base opal or creamy opal in the United States is Spencer, Idaho.[citation needed] 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, Slovakia, Hungary, Turkey, Indonesia, Brazil (in Pedro II, Piauí[22]), Honduras, Guatemala, Nicaragua and Ethiopia.
In late 2008, NASA announced that it had discovered opal deposits on Mars.[23]
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.[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 UV light. Synthetics are also generally lower in density and are often highly porous.
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.
Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.[24]
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.[25] Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2·nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.
The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.
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 -cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.
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.
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.[26]
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.[27]
References to the gem are made by Pliny the Elder. It is suggested it was adapted it 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 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.[27]
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.
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.[28] It was also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in the hand.[28][29] 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 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.[28] 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.[30]
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.[28]
Opal is considered the birthstone for people born in October or under the sign of Scorpio and Libra.