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Glossary of terms used to describe Australian opal
Precious opal - Hydrated Silica, SiO2nH2O (Silicon Dioxide) - Hardness: 5.5 to 6.5 or harder - Precious opal can be transparent (crystal opal) or opaque.
Precious opal is rare ( rarer than diamonds) and not two opals are exactly the same.
Black opal : Opal with a dark or black body.
Boulder matrix opal : A boulder matrix opal is a precious opal mixed with the parent rock rather than in seams or patches.
The parent rock is an ferruginous rock ( containing iron ), and is called ironstone.
It is found exclusively in Queensland.
It is a "Natural opal,Type 3".
Boulder opal : Precious opal having formed in cracks or cavities in brown ironstone or dark grey sandstone, found exclusively in Queensland.
Boulder opal is distinguished by the presence of the host rock on the back of the opal seam. It is a Natural Type 2 opal.
Crystal opal :Transparent or translucent opal.
Gem quality : Finest grade of opal.
Grey opal : Opal with a grey body.
Light opal : Opal with a light body and light colours.
Lightning Ridge : Mining field in New South Wales renowned for its black opal; produces also light and crystal opal.
Natural opal : Opal which has not been treated in any way other than having been sawn, preformed or cut and polished.
Opal doublet : A layer of light opal attached to a background made of black potch ( black opal without fire). Is an opal composite.
Opal inlay : Crystal opal, with the back blackened to enhance its colours, set into channels.
Pipe opal :Australian pipe opal is precious opal having formed in roundish or tubular holes in the mother rock, usually sandstone, but can be also found in
boulder.
Potch : Opal without any fire.
Rolling flash : Opal displaying colours that roll across the stone.
Semi-black opal : Opal with a dark background but not dark enough to be called black.
Solid opal : Natural Opal that has not been backed or capped in any way .
White opal : Opal with a white appearance.
