Types of Opal
Australia mines 97% of the world’s opals. The other 3% come from Mexico, Brazil and Hungary but are not the same quality that Australian Opal is world renowned for. The Australian opal fields were once an inland sea. As the ages passed and the seas receded, sea creatures were isolated and marooned, and opalised. Eventually the area dried out completely and is now dry desert country. In time the ground waters, holding silica solution, also evaporated (with some artesian springs still active deep “underground”). In a few spots they left behind the phenomenon known as 'opal'.
There are four natural types of opals found in Australia,
Coober Pedy was discovered in 1915. This is where most of the 'white' or 'milky' and crystal opals (together known as 'light opal') are mined. Coober Pedy is the main producer of white precious opal, which is predominantly seen in stores overseas, particularly in the USA. Today, the opal fields encompass an area of approximately 45 kilometres. The opal level is formed of soft pink clay mixed with soft bleached sandstone.
White opal is easier to find, occurring in most opal fields, when we cut white opal we get a yield of 50% which means that we keep 50% and we lose 50%, This ten times better than the yield we get from cutting black and boulder opal.
The next type of opal we cut is crystal opal. Crystal opal is pure hydrated silica; it is translucent so you can see straight through it. It exhibits three of the things we look for when valuing an opal. We prefer the color to be multi-directional, so you can see color from all different angles, the second thing we prefer is large blocks of color and the third thing is strong play of color that shows up in the dark.
The best way to test the play of color of the opal is to give it the “miner’s test” by holding it out of the light in the shadows. Under the table is a good place because a real gem will still shine even though it is out of the direct light.
The boulder opal is one of the rarest and most valuable forms of opal found in Australia and makes up less than 5 % of opal mined. It is very sparsely distributed through South West Queensland. It is predicted that boulder opal is going to run out in the next 10 years because of the difficulty clearing Native Title and EPA requirements of rehabilitation. It’s formed in the cracks and crevices of the ironstone boulders in gel form thousands of years ago and with the passing of centuries this jelly opal turned. Boulder opal occurs as a filling between the concentric layers or in random crevices in the ironstone. The boulder opal has a high loss factor when cutting, as we only yield 5% and has a rock waste factor of 95%. It is also the only opal suggestested to run out within the next 5 to 10 years and with the value (“it is suggested”) increasing by 15-25% each year. The boulder opal can certainly make a sound long term investment.
The most valuable and popular of all opals is Black opal. Black opal accounts for around 5% and is found at Lightning Ridge in Northern NSW. It is called Black opal because it has a black base caused by black or grey iron oxide impurities in the opal. The color bar or the ‘play of color’ of Black opal comes in all the colors of a rainbow with red being the rarest and most expensive. A Black opal is Crystal opal with iron oxide in the back. It can be grey through to very black. This black potch or common opal has no value unless we find a color bar on top of it. It’s all about the evidence we get when we cut rough opal, we find a little piece of color on the side and we take off the top layer of potch (common opal) and we hope the opal faces up. Then we take off the skin of the stone and we hope we get a face with nice color. Often it looks very nice on the side but not on the face, that is the risk we take in cutting black opal. The yield when we cut black or Boulder opal is 5% that means we lose 95% of the opal, it is very risky to buy rough black opal and cut it.
Doublet and Triplet Opal
We also have two types of enhanced opals, doublets and triplets. A doublet is a thin layer of Crystal opal with a layer of potch (black common opal) or ironstone on the back, which acts to bring out the colour of the opal. Now this black potch has value only as a backing for an opal doublet, because to make quality opal doublets, that won’t delaminate, we need to use material with the same coefficients of expansion. If we put plastic on the back of the opal (and sadly people do) to make an opal doublet, the following will happen. Plastic has a different coefficient of expansion to the opal. This means when it heats up, expands, cools down and contracts; it does so at a different rate to the opal. So the 2 different materials will delaminate and fall apart. When delaminating occurs, moisture can also come between the layers and affect the color of the opal.
Triplets are a doublet with a crystal quartz cap on the top which acts to magnify and enhance the color of the opal. The same lifetime guarantee applies to our opal triplets because they have 3 silicates with identical coefficients of expansion so they won’t delaminate. An opal triplet has a domed quartz crystal cap which has a higher refractive index than the opal. It actually kicks the colors of the opal up and because it is en-cabochon it also magnifies the pattern of the opal.
"Black opal" is a term used for opal that has a dark body color, often black or dark gray. The term is also used for opal that has a dark blue or dark green body color. The dark body color often makes the fire of black opal more obvious. ... It was mined at Lightning Ridge, Australia, the "Black Opal Capital of the World".
Boulder Opal is a unique and beautiful opal found in Queensland, Australia. It is easily identifiable because it is a mixture of ironstone and opal either in a matrix or layered. Every stone is unique and they are arguably the most affordable opal available.
"Light opal" and "white opal" are terms used for opal material that has a white, yellow or cream body color. This is the most common body color for precious opal. These stones were cut from material mined at Coober Pedy, South Australia. They are calibrated 8 x 6 millimeter cabochons.