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Types of opal12/27/2022 ![]() ![]() ![]() Australian deposits of the Coober Pedy, Andamooka, and Mintabie areas are excellent examples of sediment-hosted fields. Worldwide, deposits that contain precious opal can be divided into two major categories based on host lithologies: sediment-hosted and volcanic-hosted. Some of the best fire opal comes from Mexico. Fire opal is defined as a solid opal with transparent orange to red-orange base colour it belongs to the precious opal variety if it shows a play of colour or to the common one if it lacks play of colour. The lack of play of colours may be due to less ordered packing of the silica microspheres. The term "common opal" groups all opals without a play of colours (including the fire opal). Australia is famous for its white and black precious opal. Precious opal may be subdivided further by colour modifiers, white, black, pink, and blue, which describe the body colour of the opal. The diameter and spacing of the microspheres controls the colour range of an opal (Darragh et al., 1966). This play of colour is caused by diffraction of white light by regular packing of silica microspheres within the mineral structure. Precious opal is defined as opal with a bright, internal play of colours that may be red, orange, green or blue. The three most commonly used terms are "precious", "common", and "fire" opal. The terminology currently used to describe opal for the lapidary applications is quite complex. Opal may be deposited from silica-bearing hydrothermal solutions in hotspring or geyser environments, from meteoric waters or as accumulations of tests of silica-secreting organisms called diatoms in marine or lacustrine environments. Today, these superstitions have diminished, but some people still believe it is bad luck to wear opal. In the nineteenth century, superstitions grew about the bad luck or fate that could befall one for wearing opal if it were not the wearer's birthstone. Eastern people regarded it as sacred, and Arabs believed it fell from heaven. The early Greeks thought that opals gave their owners the powers of foresight and prophecy, and the Romans adored it as a token of hope and purity. ![]() As suggested by the derivation of its name, opal has centuries of history as a treasured gemstone. The Greek version is a modification of the ancient Indian Sanskrit name for opal "upala", which meant "precious stone". The word "opal" evolved from the Roman "opalus" or the Greek "opallios" meaning "to see a change of colour". Found by Glen, Darryl, and Carolyn Grywacheski University of Waterloo Earth Sciences Museum Collection. Despite these faults, opal's beauty is supreme and, for the past thousand years, it is recognized as a "highly prized" gemstone. Furthermore, opal from some localities, is "unstable" and may crack or self-destruct through the loss of water. Opal is brittle, sensitive to heat, and easily scratched. It is transparent to nearly opaque, and may be distinguished from chalcedony and cryptocrystalline varieties of quartz by its higher water content and lower hardness. Opal is characterized by a conchoidal fracture and it occurs in wide range of background colours. nH2O) and interstitial silica, water or CO2 gas-vapour air.Electron microscopic studies show that opal is made up of closely packed silica spheres (SiO2 Opal is hydrated amorphous silica (3% to 20% water) containing aggregates of ordered or disordered microcrystallites of a cristobalite. George Simandl, British Columbia Geological Survey, Victoria, BC Suzanne Paradis, Geological Survey of Canada, Sidney, BC. ![]()
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