OPAL MINING – White Cliffs

At White Cliffs opal is found in Lower Cretaceous age claystone belonging to the Rolling Downs Group. The claystone is of marine origin and opalised shells and fossils of fish and marine reptiles such as ichthyosaurs and plesiosaurs have been found by opal miners. The opal is not concentrated in any one horizon, but occurs spasmodically throughout the Lower Cretaceous claystone. It occurs as thin horizontal and vertical veins, as irregular nodules, as wood opal, replacing shells and as crusts on boulders.

White Cliffs is the only place where “pineapples” occur; they are a pseudomorph of Glauberite or Ikaite crystal clusters, where crystals are substituted by precious opal. This results in an object where the appearance and dimensions look like crystals, but the original mineral has been replaced by opal. They are extremely rare and valuable.

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone and carries this silica-rich solution into cracks and voids or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time and eventually opal is formed.