Australian opal is predominantly found in sedimentary rocks within the Great Artesian Basin. The Great Artesian Basin is the largest artesian basin on this planet and the main “water tank” that supplies inland Australia. The basin is the remnant of an inland sea filled with sediments. Some volcanic opals have been recovered, but production is very limited compared to sedimentary opal. Although the chemistry and the microscopic structure of opal have been well analyzed, the formation of this gemstone is still much debated within the research field.
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