The Coober Pedy Opal Fields, since their discovery in 1915, have been the largest producer of precious opal in the world, with the estimated value of production over the period 1978-86 averaging $25 million per year. Mining is now done mainly by mechanized methods in underground workings, since most of the older, shallower fields have been worked out. Opal is found in the Early Cretaceous marine Bulldog Shale, deposited on the western margin of the Great Australian (Eromanga) Basin. When fresh, the shale is a dark grey, silty and sandy, smectite-rich claystone with lenses of sand, limestone and occasional erratic boulders. Host rock for the opal is deeply weathered and bleached Bulldog Shale found throughout the Stuart Range, an east-facing escarpment and gently west-sloping tableland. In the opal fields, the weathered profile comprises bleached, porous, kaolinitic claystone (sandstone) overlying darker, denser, smectitic claystone. Precious and potch opal are found in veins infilling cracks and joints, and occasionally replacing fossils, anywhere in the upper bleached zone but preferentially from about 5 metres above to 1-2 metres below the base of this zone. Opal is usually found in near-horizontal levels marked by concentrations of gypsum, alunite, iron oxides and tubules. It is also found in the overlying Tertiary-Quaternary Russo Beds as fragments derived from erosion of veins within the Bulldog Shale. Opal was formed relatively early in the complex weathering and silicification process. The deep weathering profile and opal-bearing zone broadly follow the present topography west of the Stuart Range escarpment. Variations in the weathering environment, as reflected in the nature of the weathered profile, may have influenced opal formation. Generally speaking, all areas of deeply weathered Bulldog Shale, or of the overlying Russo Beds, have the potential to contain opal. However, the distinctive weathered profile found in known opal fields is considered the most prospective. In places, slides have provided localised control over opal formation. Known opal fields occupy only a small part of the prospective area on the Coober Pedy Precious Stones Field (PSF). Many fields have developed where levels were exposed along scarps. Potential for finding new fields is high, provided that sufficient exploration is undertaken. In the 1981 subsidised exploration programme, $30 280 was paid to subsidise the drilling of 221 shafts away from existing fields. Eleven shafts in six localities encountered opal, including four with coloured trace. One of these later became Southern Cross field. Four localities are in an area, between Hans Peak and Mars fields, which is particularly prospective. The remaining locality is east of Browns Folly field. Shafts immediately south of existing fields near Coober Pedy indicated that this area is unprospective. However, most of the southern half of the PSF is untested.