• 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 natural opal, is the most stable of all the opals in the world compared to volcanic opal which is not a stable opal and is always treated to stabilise it.
Types of Australian precious opals:
• Boulder opal: Variety of precious opal having formed in cracks or cavities in brown ironstone or dark grey sandstone, found exclusively in Queensland, Australia.
it is a 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.
Is the 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). It is an opal composite. It 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.
• 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, sometimes called "milky opal".