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.

Australian opal, being a sedimentary opal, is the most stable of all the opals in the world

Boulder opal: Variety of precious opal having formed in cracks or cavities in brown ironstone or dark grey sandstone, found exclusively in Queensland, Australia.

Quite rare opal.

Boulder opal is distinguished by the presence of the host rock on the back of a cut opal. It is a Natural Type 2 opal.

 

Boulder opal split: When the opal vein in a boulder is thick, a groove of about one millimeter deep is done around the stone into the opal seam with a diamond saw blade.The stone is then split open. It results in 2 stones instead of one, almost identical, fitting together.Depending on the size of the original stone, a pair of earrings, or a full set of earrings, pendant and a ring can be obtained with matching opal colours

 

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".

Not to be confused with Andamooka matrix from South Australia which is teated ( cooked) before polishing.

Black opal: Opal with a dark or black body.

 

Rarest and most expensive opal. Only found in Australia.

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.

Is sensitive to water. Do not get it wet or immerse in water.

 

Opal inlay: Crystal opal, with the back blackened to enhance its colours, set into channels.

 

Opal triplet: Thin slice of opal sandwiched between a supporting base and a supporting dome.

Is sensitive to water. Do not get it wet or immerse in water.

 

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.

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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.