Mintabie Opalfield, one of the most important production centres in Australia, was discovered in the early 1920s but mining only began in earnest in 1976. Precious opal is confined to Mintabie beds, a kaolinitic sandstone of presumed Ordovician Age, conformably overlying Blue Hills Sandstone. The sediments were partly silicified in the ?Miocene-Pliocene; silicification is considered to be responsible for deposition of opal in cracks and fissures of brittle rocks. Opal is found in bedding plane partings, in multiple discontinuous levels, and infilling vertical to angled joints. Top grade Mintabie opal matches, in quality and value, crystal and black opal from anywhere in Australia. Sandstone and shale of Early Cretaceous age which onlaps Mintabie beds in the southern part of the field (by analogy with occurrence elsewhere) may provide potential host rock for opal.