Australian Black Opal

Black Opal is that naturally occurring one piece or solid opal, which is jet black to dark grayish-blue or deep brown in color, and absorbs most white light impinging on it and reflects only a minimum. As a consequence, all optical diffraction effects are much more brilliant because of the sharp tonal contract.

Black Opal, a gemstone which has had an important effect overseas as a product of Australian, requires this precise meaning so that the quality of this gem can be meaningfully established. Sometimes off-colored white opal has been passed off to a visitor as being black opal!

-from "A Field Guide to Australian Opals", by Barrie O'Leary