Opal Information PDF

***Opal ***** LARGEST CONSUMERS : white opal makes the th bulk of the commercial quality opal, Europe and the US with Americans being the top consumers, China and other Southeast Asian countries (India) are the secondary top consumers MOHS SCALE: 5 to 7 VARIETY/SPECIES/GROUP: precious opal has play...

***Opal ***** LARGEST CONSUMERS : white opal makes the th bulk of the commercial quality opal, Europe and the US with Americans being the top consumers, China and other Southeast Asian countries (India) are the secondary top consumers MOHS SCALE: 5 to 7 VARIETY/SPECIES/GROUP: precious opal has play of color, common opal does not ( also know as "potch opal") COLOR/HUE RANGE/VALUE: background color is caused by impurities within opal’s silica spheres, the number or impurities dictate the how transparent or opaque the stone will be *Black opal—Background color ranges from translucent to opaque black to black, very dark gray, blue, green, or bown, but should appear black in reflected light; shows play-of-color, the darker the background the more valuable (type that is considered the most valuable, and typically sold be the piece) -Semi-black opals have a grayish background. Semi-blacks are translucent to opaque, best examples are translucent when held up to the light -Black crystal opal has a dark background, but it’s transparent to semitransparent rather than opaque. -Gray-base black opal also has a gray background, but it’s opaque when held up to light *White opal—Background color ranges from translucent white to medium gray background; shows play-ofcolor, commercial qualities in pale and cloudy yellows or browns with faint, scattered play-of-color as well as top-quality almost transparent gems featuring distinct patterns of crisp, lively play-of-color. *Crystal opal—Background ranges from transparent to semitransparent; shows exceptional play-of-color Semi-crystal opal is semitransparent with a light background color, The term “crystal” refers to its clarity *Water opal—Background ranges from transparent to translucent; shows faint play-of-color or no play-ofcolor at all making it low in value, also known as jelly opal, colorless, white, purple, brownish, blue, or bluegreen in hue, with a hue light in tone *Boulder opal—Includes host-rock fragments, or matrix, as part of the finished gem; shows play-of-color, thin layers of precious opal cut to include the sandstone or ironstone matrix, which makes it more durable then other types, found in veins or within matrix and background color can be dark or light, -"Boulder Matrix Opal" combination of opal and ironstone where opal is mixed throughout the ironstone, -Boulder black opal has an ironstone matrix -Boulder white opal might describe boulder opal with a sandstone matrix *Fire opal—Background color ranges from transparent to translucent reds and oranges to yellows; might or might not show play-of-color, color is caused by iron oxides impurities, top color is usually red-orange to bright red, cloudy look = drying out and may crack -Cherry Opal is a type of fire opal that lacks play of color, background color that gives cherry opal its name is a transparent red-orange to bright red. *Assembled opal—Precious opal layers, or layers of precious opal and other material, cemented together to improve durability and appearance, -Doublets are cemented to obsidian, dyed black chalcedony, black glass, natural potch opal, or plastic. -Triplet is a thin layer of opal cemented between a domed top of colorless quartz or clear glass and a backing of obsidian, chalcedony, or black glass, graded on 3 quality levels based on opal layer an construction: Intense play of color(including red) + assembly, less play of color but still attractive, Blue/ Green play of color with poor assembly and some areas lacking POC COLOR/HUE RANGE/VALUE conti..: *Other opal types—Gem-quality, non-phenomenal; occur in pink, blue, and other colors, Gem-quality nonphenomenal opal is also found in green and yellow, as well as in neutral colors from cream to brown. with high to low saturation, opaque to semitransparent with varying patterns/attractive inclusions or intergrowths of another mineral—chalcedony—give apicturesque agate-like appearance CHEMICAL COMPOSTION: hydrous silica ( or Silica consisting of silicon & oxygen) FORMING SOURCE: sedimentary and volcanic rocks, 4 theories are recognized; weathering theory: rain water, the microbe theory:enzymes and acids secreted by tiny organism, syntectonic theory:silica-saturated solutions are deposited, and redox theory: 100 million years ago of a cold, oxygen-poor inland sea. Pink opal, beyond its use as a gem, has been studied extensively by geologists and gemologists seeking a better understanding of opal formation. CRYSTAL STRUCTURE: no over all crystal structure, but instead three-dimensional arrangement of uniform, submicroscopic silica spheres, the uniformity of there arrangement and size of the spheres effects the play- of-color POSSIBLE PHENOMENA: play-of-color, the size of the spheres determines what hue POC is displayed, experts call an opal’s vividness “brilliance” and its play-of-color “fire,” POC color ranges is also important, “multicolors”—the play-of-color consists of just one main color and two or more secondary colors, Value: Red is the most preferred hue followed by orange then green, the variation of what a direction a hue is visible can make it hard to determine the dominate hue Pattern: pattern is the arrangement of an opal’s play-of-color, Industry general categories: Pinfire—Very small patches or “dots” of play-of-color Flash—Large areas of play-of-color Harlequin—Large, distinct, usually rectangular patches of play-of-color with edges touching each other, and considered to be the most valuable "Contra Luz" is a type of opal where play of color is only seen under reflected light CLARIETY CHARACTERISTIC: opaque to transparent with value varying depending on type, extinction or "dead spot" where no play of color is visible, cloudy or milky background color lowers the value of any opal and make indicate lack of stability, surface blemishes such as fractures and pits, when dried out "crazing" can happen which are a network of cracks resembling a spider web COMMON TREATMENTS: sugar treatment or smoke treatment, impregnation, dying, fracture filling, coating, An opal must be fairly porous in order for these treatments to work. Both sugar and smoke are used to darken the background resulting in the play of color being brighter, Impregnation - filled with melted wax, resin, polymer, or plastic, if the impregnation is colored its labeled impregnated and dyed, Ethiopian opals are often just dyed, fracture filling is done to correct/ prevent crazing, coating with resin or plastic is done to improve luster COMMON CUTS: Cabochon, fire opal is often faceted unless it has play-of-color then its a cabochon, "Other" Opals are often fashioned into oval or round cabochons or polished as free-forms or carvings, with carat range over a hundred , commercial quality are typically calibrated, maximizing play of color is a cutters main goal , with common opals color and weight retention are the most important factors TRADES NAMES: "Pandora, "Light of the World", "Empress" COUNTRIES OF ORIGIN: Australia is the worlds major opal supplier, along with Ethiopia and Mexico, other location include Brazil, the US, Canada, Honduras,Peru, Madagascar, Slovakian, and Austria, Fire Opal is often found in Mexico, Honduras and in the southwestern United States. Australia: Rich deposits of black, white, crystal, and boulder opal are found around the Great Artesian Basin (GAB), New South Wales town of Lightning Ridge is the world’s main source of fine-quality black opal (commerical mining began there in 1910) White Cliffs, located in northwest New South Wales, was Australia’s first commercial opal field (before black opal) South Australia also hosts several large white opal deposits, including Coober Pedy, Andamooka, Mintabie, and Lambina, Coober Pedy is the largest opal producer in terms of volume, Lambina is the newest mine, -Boulder opal is found in iron stone matrix in Queensland in the the cities of Koroit, Yowah, and Quilpie specimens are famously known as “Yowah nuts” , Queens also produces crystal opal Ethiopia: opals found in central Ethiopia are volcanic in origin, discovered 1993 at Yita Ridge in Shewa Province, white , crystal and brown, in 2008 in the town Wegel Tena, in Wollo Province and make up the majority of Ethiopian opals on the market, Wollo’s opals shows unusual diffraction patterns, with each color patch displaying a small rainbow. This deposit also yields a very special “digit pattern” opal, made up of intersecting segments of precious and common opal. Wollo opals are consider to be the most durable, the countries opal production is dominated by small-scale artisanal operations. Mexico- the building blocks of red-to-yellow fire opals from Mexico are randomly arranged silica platelets rather than spheres, sourced from central Mexico’s volcanic fields, the state of Querétaro and the city of Magdalena in Sonora are the two main sources with mining active since 1870 (with peak production in the late 1960s and early 1970s at Querétaro’s famous Santa Maria del Iris mine,) 3 principle opal producing cities in the state of Querétaro are Tequisquiapan, San Juan del Rio, and Colon, 10 other states in Mexico produce opal, including Chihuahua, San Luis Potosí, Guerrero, Hidalgo, and others. Other locations: Brazil in the town of Pedro Segundo in the state of Piauí, In the US western Oregon, Spencer in Idaho and northwest Nevada, Madagascar is also known as a source of opal, especially fire opal, Small quantities of gem-quality opals are also found in Honduras, Peru, Venezuela, Kazakhstan, Turkey, Slovakia, Austria, Tanzania, and France, Both US and Madagascar opals are from volcanic sources. MAIN SYNTHETIC PROCESS: Gilson Created Opal - used chemical precipitation to manufacture submicroscopic silica spheres,under magnification a structural pattern can be seen resembling honeycomb or snake skin, 1990's Japanese manufactures started producing plastic impregnated synthetic opals COMMON IMITATION MATERIAL: Slocum Stone, an opal imitation made of glass (similar to play-ofcolor, apparently caused by tinsel-like flakes.)Plastic imitation opal was first marketed in Japan in the late 1980s with play of color made of acrylic resin and polystyrene "Opalite" and "Opal Essence" are two trade names for this type of imitation opal. CUTTING/TRADING CENTER: Santiago de Querétaro is Mexico’s most important opal cutting and trading cneter centers, Jaipur, India, is one of the main fashioning hubs for Ethiopian opal as well as fire opal from Mexico,China was a major cutting hub, but other countries such as India, Vietnam, and Thailand have also been used as they were more ecomonical historically Japanese consumers were the top buyers of opal back in 1980's - 1990's