Mapping of the Coober Pedy Opalfield has shown that opal occurrence, although erratic, is not entirely unpredictable. Several groups of workings are present in an area of about 150 square miles. The host rock is a distinctive pink and white sandstone, but at each group of workings opal is localised in a different structural setting. Depressions in the host bed, caused by minor faults, are the control on one field. At the remainder the control is still undefined, but undulations in the sandstone are probably significant. If the base of the sandstone can be determined by geophysical techniques this will greatly aid prospecting in the region. Opal formation is a late phase of the Pliocene laterite forming process, and a genetic cycle involving entrapped supergene water is proposed. Past production, present activity and mining methods are summarised. Results of petrological work and measurements of host rock pH are presented.