7/27/2023 0 Comments Opal color![]() Opal was first mined commercially at Listowel Downs in Queensland in 1875 and later at White Cliffs in NSW. See Google Arts and Culture - Australian opals or Australian National Gemstone for further information. Opal is found around the world (Brazil, Mexico, Honduras and the western US) however Australia produces 95% of the world's precious opal and it is our official national gemstone. Probably the most famous opalised fossil is Eric the Pliosaur (Cretaceous age marine vertebrate) which was found at Coober Pedy and now forms part of the Australian Museum collection. At Lightning Ridge in NSW, small opalised dinosaurs and primitive early mammalian remains, together with shallow marine shellfish and crustaceans have been found. Opals are frequently layered and if a rare red layer is present it is at the base in the thinnest portion of the vein and indicates that gravity played a part in the arrangement of the silica spheres.Īustralia is the only part of the world where opalised animal and plant fossils have been found. Impermeable barriers between the sandstone and the underlying rocks trapped the silica-carrying groundwater where it slowly hardened into a gel forming opal in veins and lenses. Small faults and joints in the rocks formed pathways for movement of the groundwater as it penetrated downwards. These sedimentary rocks were deeply weathered and this weathering released silica into the groundwater. In Australia, precious opal is found in Cretaceous age sandstones and mudstones. ![]() This started an 'opal rush' and soon the settlement of the Stuart Range Opal Field was founded, better known now as Coober Pedy and together with Lightning Ridge in NSW, the bulk of the world's opal continues to be produced. Making camp one night, a 14 year old boy found an opal. In 1915 a group of people were prospecting for gold at the edge of the Great Victoria Desert northwest of Adelaide. Gem-grade precious Ethiopian Welo opal pendant. There are many aboriginal dreamtime stories that feature opal.Īustralian opals discovered during the late 1800's found little favour with European markets but their commercial value increased in the 1900's and in 1932 Australia took over as the major producer of opals in the world and remains the largest producer to this day. As early as 250 BC the Romans prized opals, thought to have come from mines in Eastern Europe, the ancient world's main source of opals. Opal artefacts several thousands of years old have been discovered in East Africa. Opals are used in jewellery and ornaments. There is some uncertainty around the origin of the word opal - it may come from the Greek opallos meaning 'to see a change (of colour)' or may be Sanskrit for ‘(precious) stone’. Boulder opals are cut with the natural host rock, ironstone, on the back. Black opals (very rare and valuable) have a dark background and colours ranging from brilliant red through to greens, blues and purples. White opals have delicate, pale colours on a lighter background. Larger spheres provide all colours, smaller ones only blues and greens. Opals that have a predominantly red colour are very rare as they only occur where larger silica spheres were deposited. The spheres diffract white light, breaking it up into the colours of the spectrum. Opals are multi-coloured and consist of small spheres of silica arranged in a regular pattern, with water between the spheres. (Close-up images of inclusions by Wimon Manorotkul.Idealized molecular structure of precious opal: an orderly array of silicon dioxide spheres. This Mexican fire opal has all the characteristics of a unique and beautiful gemstone and is a wonderful addition to the National Gem Collection. ![]() With magnification, looking inside the opal you can see interesting inclusions: "stalagmite and stalactite" formations that are probably goethite rods, along with what appear to be hematite crystal inclusions. ![]() ![]() This fire opal cabochon is semi-transparent with an orange body color and displays beautiful green, yellow, orange and red play-of-color. However, due to their transparency, many fire opals are faceted. Opals are typically cut en cabochon or polished free-form to best show the play-of-color. Fire opals are commonly referred to as Mexican opals because most of the finest fire opals are found in Mexico. The play-of-color in opal is due to the orderly arrangement of these spheres acting like a diffraction grating, breaking visible white light in to separate colors. Gem opal consists of tiny silica spheres tightly packed together the voids or spaces between the spheres contain air or water. The body color is caused by inclusions of iron oxides. Fire opals are transparent to semi-transparent, resembling gelatin, with a red, orange, or yellow body color, with or without play-of-color. ![]()
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