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Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal consists of small silica spheres, of uniform size and arranged in a regular pattern. The colour in precious opal is caused by the regular array of silica spheres diffracting white light and breaking it up into the colours of the spectrum. The play of colour in opal depends on the angle of incidence of the light and can change or disappear when the gem is rotated. The size and spacing of the spheres also controls the colour range of opal. In opals showing red flashes, the spheres are larger than those showing violet or green flashes. In potch opal and common opal the silica spheres may be absent or too small or irregularly arranged to produce colour. 

“Opal is one of the world’s most fascinating gemstones, its brilliant play of colours- changing and flashing as the stone is turned- sets it apart from all others. No two opals are ever exactly the same: each has an individual pattern, and there is no end to the variety available. A single opal can display a range of colours encompassing the entire visible spectrum”.

Opal at Andamooka occurs in the shallow marine Bulldog Shale, part of the Marree Subgroup of Early Cretaceous age, which overlies Algebuckina Sandstone or laps directly onto pre-Mesozoic rocks. The top sub-unit of Bulldog Shale, called kopi by the miners, consists of highly weathered white sandy clay with scattered, large erratic boulders. At the base of the kopi there is an extensive sandy boulder bed, called the concrete or conglomerate band, which contains numerous pebbles, cobbles and boulders of pre-Mesozoic rocks, chiefly Arcoona Quartzite. Beneath the conglomerate band is a pale brown, grey or yellow claystone with a low sand content referred to as the mud. Opal at Andamooka occurs predominantly at one horizon, referred to by miners as the level, at the contact of the conglomerate band and the mud. Other sub-levels (squibby levels) occur above the main level but none are as prominent, continuous or productive. The main opal varieties produced are crystal opal (transparent to translucent), white opal, and some black opal. Painted ladies are boulders, generally of quartzite, which split along a fracture to reveal a coating of opal. Matrix opal is cloudy stone with flashes of colour, which is thought to form by replacement of limestone boulders in the conglomerate band. Opalised sandstone or opalstone forms by deposition of opal in the spaces between the quartz grains in sandstone boulders. Both matrix and opal sandstone can be treated by soaking in sugar solution and boiling in sulphuric acid to darken the body colour and enhance the play of colour.

 

South Australian Museum

  1. Illuviation features specific to and characteristic of pedogenic silcretes vary from metre scale in outcrop to sub-millimetre scale in pores and channels in thin section. Cappings above nodules and columns are alternating coarse- and fine-grained laminae. Void deposits are opal enriched in titanium oxide (Andamooka and Stuart Creek opal fields)