Australian "Boulder" Opal Watches - The Story of Where "Boulder" Opal Fits in the Range of Opal = 60 to 120 Million years old
Posted by Mark Poole on
Types of Opal
- Black Opal, Semi-black Opal, (Boulder Opal shown above), Crystal Opal, White 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'.
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 white opal is cut a yield of 50% which means that 50% is Kept and 50% is lost, This is ten times better than the yield from cutting black and boulder opal.
Crystal opal is pure hydrated silica; it is translucent so you can see straight through it. It exhibits three of the things that are look for when valuing an opal. Prefered color to be multi-directional, so you can see color from all different angles, the second thing prefered 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.
Boulder Opal (Photo Attached to this Blog)
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, the yield is only 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 you find a color bar on top of it. It’s all about the evidence you get when you cut rough opal, you find a little piece of color on the side and you take off the top layer of potch (common opal) and you hope the opal faces up. Then you take off the skin of the stone and you hope you get a face with nice color. Often it looks very nice on the side but not on the face, that is the risk you take in cutting black opal. The yield when you cut black or Boulder opal is 5% that means you lose 95% of the opal, it is very risky to buy rough black opal and cut it.