About: Opals
Opal is one of the world’s most beautiful gemstones. Unlike any other gemstone, opal displays rainbow spectrum colours in a moving pattern known as the “play of colour”. Opal is Australia’s national gemstone - over 95% of the world’s precious opal comes from Australia.
Opal forms as a mineral solidified from gelatinous or liquid silica (silicon dioxide and water) deposited in cracks and cavities left by decaying vegetation, wood, crustaceans and bones (in sedimentary and volcanic areas) over millions of years. By filling in cavities, regardless of their shape, opal is formed as pseudomorphs (materials with shapes that are unrelated to the chemical content). The most common are opalised wood and seashells.
There are several varieties of opal found in nature and these fall broadly into two categories - precious opal and common opal (potch). Precious opal exhibits the spectrum display whereas common opal does not. About 95% of all opal mined from the opal fields is common or potch. Generally opals with a black or dark body tone are more valuable than those with a white, light, or crystal body tone. Black opal from Lightning Ridge (NSW) is the most prized opal and may realise prices over $10,000 a carat.
Lightning Ridge is also the only opal field in Australia with fossils of diverse land-living organisms - pinecones and platypuses, microscopic protozoans and gigantic dinosaurs. The fossils are usually exact replicas of plant, shell or bone material, and at times they are comprised of gem quality black opal, which is as valuable as diamonds and more beautiful.
Other famous Opal deposits in Australia include Andamooka (SA), Coober Pedy (SA), Mintabie (SA), White Cliffs (NSW), Bull Creek (QLD), Hungerford (QLD), Opalton (QLD), Opalville (QLD), Quilpie (QLD) and Yowah (QLD).
SPECIFICATIONS:
Hardness - 5.5 to 6.5
Refractive Index - 1.44 - 1.47
Specific Gravity - 1.95 - 2.20
Lustre - Vitreous to pearly
Streak - White
Fracture - Conchoidal
Transparency - Transparent to translucent
Crystal System - N/A as opals are amorphous
Cleavage - Absent
TYPES OF PRECIOUS OPAL:
Black/Semi Black Opal
This opal is solid with a black or very dark colour and is the most rare and valuable of all opal, due to the colours appearing brighter and sharper and the “play of colour” occuring brilliantly. Most black/semi black opal is mined from Lightning Ridge, with some from Mintabie (SA) and Coober Pedy.
This opal is solid with a black or very dark colour and is the most rare and valuable of all opal, due to the colours appearing brighter and sharper and the “play of colour” occuring brilliantly. Most black/semi black opal is mined from Lightning Ridge, with some from Mintabie (SA) and Coober Pedy.
White Opal
This opal has a white or light colour and an opaque background and comes mostly from Mintabie and Coober Pedy. White opals appear “milky”.
Crystal Opal
Is translucent or transparent with sharp colours visible below the surface. Can be light or dark and the more transparent it is, the more valuable it is.
Is translucent or transparent with sharp colours visible below the surface. Can be light or dark and the more transparent it is, the more valuable it is.
Matrix Opal
A porous opal, light in colour. When it is cut, polished and carbonised (by treating it with sugar and sulphuric acid) the end result looks like a black opal. Matrix opal has a salt and pepper look and can be formed naturally where silica runs into small cracks and crazing in ironstone boulders (this is known as a “Boulder Matrix”).
A porous opal, light in colour. When it is cut, polished and carbonised (by treating it with sugar and sulphuric acid) the end result looks like a black opal. Matrix opal has a salt and pepper look and can be formed naturally where silica runs into small cracks and crazing in ironstone boulders (this is known as a “Boulder Matrix”).
Boulder Opal
A naturally formed solid opal which consists of fine layers of opal deposited by nature on and in fissures of ironstone. A dark background colour gives it the appearance of black opal. Usually forms in irregular shapes. Mostly found in Queensland and Andamooka (SA).
A naturally formed solid opal which consists of fine layers of opal deposited by nature on and in fissures of ironstone. A dark background colour gives it the appearance of black opal. Usually forms in irregular shapes. Mostly found in Queensland and Andamooka (SA).
Fire Opal
This opal is a clear orange crystal opal and is usually mined in Mexico. Most fire opals have an orange or reddish base and are used for faceting.
This opal is a clear orange crystal opal and is usually mined in Mexico. Most fire opals have an orange or reddish base and are used for faceting.
Yowah Nuts
These are found in Yowah, western QLD. An opal deposit forms inside a nut-like kernel of ironstone, creating an unusual appearance. Yowah nuts are very hard and strong and can be polished to a high gloss finish.
These are found in Yowah, western QLD. An opal deposit forms inside a nut-like kernel of ironstone, creating an unusual appearance. Yowah nuts are very hard and strong and can be polished to a high gloss finish.
Doublet Opal
A doublet opal is made by bonding a layer or slice of natural opal to a dark backing. The backing can be potch, black glass or ironstone boulder. The dark backing enhances the colours, making them appear brighter. As only a small slice of natural opal is used, these are cheap to produce.
A doublet opal is made by bonding a layer or slice of natural opal to a dark backing. The backing can be potch, black glass or ironstone boulder. The dark backing enhances the colours, making them appear brighter. As only a small slice of natural opal is used, these are cheap to produce.
Triplet Opal
Made like the doublet opal. except a crystal or glass dome is placed over the slice to protect the opal. Quality ranges from souvenir to gem quality and is produced at a low cost. Best specimens show brilliant colours and a “play of colour” like black opal.
DID YOU KNOW:
Made like the doublet opal. except a crystal or glass dome is placed over the slice to protect the opal. Quality ranges from souvenir to gem quality and is produced at a low cost. Best specimens show brilliant colours and a “play of colour” like black opal.
DID YOU KNOW:
Opals are measured by carat weight and sometimes are worth more than diamond. Opal comes from many ancient words - UMPALA (Sanskrit) meaning “precious stone”, OPTHALMUS and OPALUS (Greek) meaning “change in colour” or “eye” and PAEDERES (Greek) meaning “child” or “favourite”.
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