Black Opal
The Black Opal is a naturally occurring one-piece or solid opal that is jet black to dark grayish-blue or deep brown in color. This opal 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 contrast.
The Black Opal, a gemstone which has had an important effect overseas as a product of Australia, requires this precise meaning so that the quality of this gem can be meaningfully established. Sometimes an off-colored white opal can be passed off to a visitor as being a black opal!
These points can be considered in regards to recognizing a genuine black opal:
Black refers to the body color of a natural, solid, and precious opal. Sometimes a transparent layer of precious crystal opal will naturally form on black potch opals and may transmit that base's darkness through its own substance.
Whether naturally black or artificially stained, the Matrix Opal and the Queensland Boulder Opal can’t be described as a black opal.
No opal doublet should be described as a black opal, even though the veneer of noble opal may have come from a black opal.
The categories of black, semi-black, and light-to-grey opals cannot be inflexibly defined. When does a gemstone grade from black to semi-black? Your common sense can dictate this. If you’re in doubt, put it in the lighter category.
Why we might ask,
is a black opal colored black?
The reason for blackness in volcanic opal is the presence of impurities of iron oxides, scattered like fine dust through the substance, in a sufficient quantity to create a jettiness of color. A black opal from Lightning Ridge has carbon along the pseudo crystalline boundaries. The base color of a white opal is a property of the structural imperfections in the stacking arrangements of the basic silica microspheres that compose opal. These imperfections scatter and diffract white light. Black opals absorb most of the white light that impinge upon it. That fraction is diffracted as glorious colors.
*The “About” text is from A Field Guide to Australian Opals by Barrie O'Leary.