Australia’s Great Artesian Basin (GSB) hosts widespread deposits of Opal, and continue to be the major source of high-quality gem opal for the international coloured gemstone market. Opal mining in the GAB has produce billions of dollars’ worth of exquisite gems, including light opal mainly found in the opal fields of South Australia, Boulder Opal minded in central Queensland, and black Opal mainly mined around the town of Lightning Ridge.
Opal in GAB deposits comprises both common potch and rare precious opal, and occurs as replacements of fossils as in filling in ironstone concretions and ferrierites and as tectonically-generated fault and fracture-controlled opal veins. Owing to the numerous varieties of opal, which occur in the sedimentary host rocks the GAB, a number of contrasting theories have been postulated over the years to try to explain the formation of these important coloured gemstone deposits.
