![]() The following is a web-based list of the many terms that have been applied: A few of the terms, such Fire Opal and Hyalite, have received universal endorsement, but most are isolated names that have appeared only a few times in published reports or catalogs. Opal is plagued by a surfeit of prefixes and descriptive terms. Strongly fluorescent and very aesthetic hyalites were recently discovered in Tarcal, Hungary. World-class hyalite specimens are known from Valeč, Czech Republic. Other sites include Menze Gishe in Ethiopia, Piaui in Brazil Gracious and O Dios in Honduras. Canadian precious opals come from Vernon, British Columbia. Queretaro (Queretaro State) and Magdalena (Jalisco State) in Mexico are world-class sites for fire opal. ![]() Many precious opal localities are located in US: Last Chance, Kern County (California) Spencer area, Clark County (Idaho) Virgin Valley, Humboldt County (Nevada) and Opal Butte, Morrow County (Oregon). Opal mines in Coober Pedy, South Australia. There is no problem if the origin and treatment of the opals is fully disclosed - which is often not the case, unfortunately. Many even sell wet hydrophane as a precious opal. Many dealers soak their opals to hide the crazing effects and lower the risk of crazing of low quality opals. Various precious opal frauds and scams are often present in the business, ranging from disreputable treatment and fake labels (opals from some localities are less prone to crazing) and extending to oiled, nail-polished or even completely faked opals. Opal is generally not confused with any other mineral except chalcedony, a cryptocrystalline form of silica with a noticeably greater hardness (6.0 - 7.0). To preserve the specimens intact, collectors need to avoid drying of opals or sudden change in temperature or light. This naturally reduces their susceptibility to crazing, but makes it equally hard to present them elegantly for viewing. The crazing problem results in the inconvenient remedy of storing opal specimens underwater. This is one of the reasons for the very high price of true Australian precious opals. Some localities produce high quality and very resistant opals while some others produce nice opals very prone to crazing. So the overall chance of crazing depends mostly on water content and internal microstructure. One of the obvious reasons of crazing is a loss of water. Precious opal from Quilpie, Queensland, Australia. Its fracture pattern is typically conchoidal, but may be splintery, uneven or irregular. The mineral is vitreous, but may be pearly, resinous or waxy variable in hardness (4.5 - 6.5), with a white streak, and a density between 1.98 and 2.25. Banded and multicolored varieties are common. The color of opal covers the visible spectrum: Colorless, white, yellow, orange, red, purple, blue, green, gray, brown, and black. Its usual habits run the gamut, including massive, botryoidal, nodular, earthy, reniform, vein-like and crusty. Opal is well-known for mimicking the forms of other minerals, as well as wood, shell or bone as pseudomorphs. This process takes decades in human-made glass and millions of years in opals. The amorphous forms of silica tend to internally recrystallize into more stable forms. Photo: Zbyněk Buřivalĭiagenesis - the gradual sedimentary burial of rocks under heat and pressure - is believed to produce gradual transitions from the non-crystalline to microcrystalline forms of opal. And in the early 20 th century, Russians could believe that catching sight of an opal among items for sale was a reason to avoid making further purchases, for fear of provoking the evil eye.Ĭommon (milky) opal with black dendrites from Bohouškovice, Czech Republic. On the other hand, the publication of Sir Walter Scott's Anne of Geierstein, in which an opal possessing supernatural powers is rendered colorless and impotent upon contact with holy water, caused a precipitous 50% drop in European sales of opal for the next 20 years. ![]() Under bizarre circumstances, some believed it conferred invisibility on the wearer. ![]() ![]() For example, during the Middle Ages, any opal that displayed the spectrum of colors associated with all other gemstones was considered an exceptionally lucky stone. The value placed on opal has fluctuated significantly, based on contemporary cultural beliefs. Opal is an exception from IMA rules and it is classified as a true mineral. Nevertheless, nearly all historic reference sources classified it as a mineral, a category justified in part because some constituents of opal are now known to be microcrystalline forms of silica. Lacking a true crystal structure, opal is a mineraloid. These attributes have guaranteed its continuing value as a gemstone since pre-Roman times. Opal is most valued for the rich variety of colors it exhibits, their interplay in ordinary light, and their unusual range of fluorescence. ![]()
0 Comments
Leave a Reply. |