Australian black opals are the most valuable of all the opals. They are found on one place only. Lightening Ridge in northern New South Wales. Millions of dollars of black opals have been found in Lightning Ridge and it is likely that millions more will be found also.
A black opal color can range from dark gray to jet black but this does not refer to the spectral colors in the opal, rather to the general body tone. The spectral colors can still be seen in the depths of the opal and can look even more striking due to the dark tone of the opal .
Black opals have a high carbon and iron oxide content within the stone which give the stone its darkness. It also makes it extremely rare and very expensive. A top of the range black opal can cost over 10,000 US dollars per carat making them much more expensive than diamonds!
There are also semi black opals. These are also known as dark opals or gray opals. They have a darker tone than the white but not so dark they could be called black opals. These are mostly found at Lightning Ridge but have also been found in Mintabie in South Australia as well. A semi Black can fetch over 2,000 US per carat.