What is an Australian fairy opal?
• Fairy opal is the term given by opal miners in Queensland, to treated opalized sandstone.
• Opalised sandstone is sandstone ( mother rock of the opal) impregnated with small grains of precious opal.
• Opalised sandstone is only found in Queensland Australia.
• Opalised sandstone is rather localised and when found, no opal of significant size is present close by.
• It appears that the opal formation has been limited and restricted to the sandstone in the area.
• It is not particularly common, and opal colours can vary from blue to all the colours of the rainbow ( the latter being rather rare).
• Part of treating opalized sandstone is to darken the sandstone which as a result make the opal colours stand out looking very bright.
• The other part of the treatment is to make the stone harder without any voids ( as natural sandstone is porous),
which allow to highly polish it.
• There are several methods of treating opalised sandstone:
• It is cooked in a saturated solution of sugar or honey, wether dissolved in water or in household cooking oil, until the sugar or honey is carbonised.
• The contrast between the mother rock and the opal brings out and enhance the colours of the opal ( the opal itself is not affected by the cooking process) and it results in a black matrix kind in appearance.
• It is common practice to preform the stones before cooking because the blackening of the rock is only a couple of millimeters deep and the final processing such as sanding and polishing can only be done on the blackened part of the stone; otherwise, the non blackened part will reappear and the full treatment will have to be done again.
• Cut and polished Fairy opal can be fairly valuable based on the beauty and rarety of the finished product.
• Fairy opal is considered to be a treated opal matrix.
• Queensland Fairy opal is in some ways, similar to treated Andamooka matrix stone from South Australia, but the nature of the mother rock is different.