It is coming almost exclusively from one place: Lightning Ridge Australia. It was found at the end of 19th century and the mining soon started in 1901. Black opal bearing layer is composed of Finch clay stone. Buried 3 to 30 meters below the surface, over a 100 million years ago.
To reach the gems, first a vertical shaft must be sunk and then, the digging follows the opal rich layer horizontally. The crushed dirt is then transported outside to be mechanically sorted and washed. The last phase is manual sorting. Even with all the machinery we have available today, mining for black opal is still a hard and dangerous work. With the rising fuel cost and tighter government regulations, stream of quality black opal becomes ever smaller.
Black Opal is found in seams, veins, pipes and various nodules, called nobbies. Opal was deposited by silica bearing water that penetrated the rock to deposit opal inside cracks and other empty cavities. Many of them left by remains of flora and fauna of Cretaceous Period. This results in a large number of opal fossils being found in Lightning Ridge area. The most precious kind of fossils indeed.
Black Opal can show any colour there is. Blue and green being most common and red, traditionally rarest and most expensive. The price is affected by colours it shows, their strength, pattern (some are much rarer than others), directionality (amount of colours visible at different angles), the body colour (the darker the better), and the size of particular gem of course.
Rough opal can be found in a variety of shapes. With the black opal being particularly expensive, it is sometimes carved rather than cut into calibrated sizes.This is done mainly to preserve the weight but as a side effect we can enjoy unique artistic creations in this most worthy medium. In my opinion this is a very good choice. Unlike many gemstones, opal doesn’t need precise geometrical form to show its fire. It already is the most unique gemstone, so the unique shape matches its character well.
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