How does opal form?
While small differences do exist between the geology of individual opalfields, each of the Australian commercial opal fields share some common features: • A source of silica derived by protracted chemical weathering of the sandstones of the Great Artesian Basin. • An impermeable or semi impermeable stratum, underlying the weathered sandstone strata of the Great Artesian Basin, that traps downwards-percolating silica. This permeability barrier is provided by a claystone layer in the New South Wales, a similar strata in South Australian opal fields, and a layer of ferruginous ironstone in the Queensland boulder opal fields. • Passageways (joints and faults) within the sandstone strata that allow the downwards flow of silica-rich solutions and their entrapment at an underlying permeability barrier. Subsequent slow evaporation of water from the trapped silica slowly allows the formation of silica spheres which flocculate into a mass of three dimensionally stacked structures of silica that
