The Great Artesian Basin is one of the best known geological structures in Australia. It is an extensive sedimentary basin extending from the Gulf of Carpentaria in the north to a latitude of about 31° in the south (a distance of some 1600-1700 km), and from about longitude 151° in eastern Queensland to longitude 134° in South Australia and the Northern Territory, a distance of some 1800 km.

R.C. Sprigg (R1640) describes it as:

It is an extensive area of low relief, mostly about 150 m above sea level, rising rarely to 400 m, and, in the region of Lake Eyre (usually a large, dry salt pan) falling to about 12 m below sea level. In some areas spectacular scenery is developed by erosion to form vast panoramas of residual mesas and similar monadnocks. Around the edges can be found large areas of dry, barren country, often covered with stones ('gibbers').

The Great Artesian Basin is a major aquifer and yields very large amounts of artesian and sub-artesian water, although the wide use of this resource for stock in arid areas has resulted in lowering of pressure in the aquifers. The recharge is from the eastern side of the basin along the western slopes of the Great Dividing Range in Queensland and New South Wales.

The basement rocks over most of the area are Precambrian, although in some areas they may be early or late Paleozoic. The basin began to form, possibly in the late Permian, certainly in the early Mesozoic, extending over a large part of the central continent by Cretaceous times. In the central and northern parts of the basin, the sediments attain a thickness of the order of 2000 m, mainly Cretaceous in age. The earlier beds are of marine sediments, but towards the end of the Cretaceous, as the basin began to shrink in area, lacustrine sediments were deposited.

These beds are well exposed around the edges of the basin, and are the host rocks for the major opal fields. They are located mainly along the southern boundaries of the basin, although, in south western Queensland they extend further into the central areas. There are no doubt many opal-bearing areas to be discovered, but with limited surface indications and sporadic occurrence of the gem, exploration tends to be a costly and hazardous business. No reliable methods for identifying buried opal-bearing deposits have yet been devised, although at least one major company is trying to solve this problem.

The general geology and mode of occurrence is similar in all of the active mining areas, but differs in detail. The geology is outlined in the next few sections for each of the major opal fields, as well as the occurrence of the gemstone, the manner of which tends to be characteristic for each field. The data has largely been obtained from information published by the Department of Mines (or equivalent department) in the three opal mining States, South Australia, New South Wales and Queensland.