Black Opal is the most prized, beautiful and sought after type of opal in the world. Coming almost exclusively from one place: Lightning Ridge Australia. It was found in the end of 19th century and the mining soon started in 1901. Black opal layer is composed of Finch claystone, burried 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 for the last phase of manual sorting. Even with all the machinery we have available today, mining for black opal is still a hard and dangerous work and 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. Curiously opal was deposited by silica bearing water that penetrated the rock to deposit opal inside cracks and any other empty cavities. Many of them were caused by remains of flora and fauna of Cretacreous 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, with blue and green being most common in Australian opals 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. Opal can be found in a variety of shapes and the black opal is particulary expensive, so sometimes it is carved rather than cut into calibrated sizes, to preserve its beauty. In my opinion this is a very good choice as 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.