What is Black Opal?
Black Opal is that naturally occurring one piece or solid opal whish 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!
These following points can be considered in the problem of recognizing a genuine black opal:
Black refers to the body color of natural, solid, precious opal. A clear transparent layer of precious crystal opal naturally formed on black potch opal may transmit that base's darkness through its own substance and so assume the quality of being black. This is black opal, too.
Black Opal is not a term applicable to matrix opal, whether naturally black or artificially stained, nor to Queensland boulder opal.
No opal doublet should be described as 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 opal cannot be inflexibly defined. When does a stone grade from black to semi-black? Your commonsense can dictate this and, if in doubt, put it in the lighter category.
Why, we might ask, is black opal black? The reason for blackness in volcanic opal is the presence of impurities of iron oxides, scattered like fine dust through the substance, in sufficient quantity to impart a jettiness of color. Black opal from Lightning Ridge has carbon along the pseudocrystalline boundaries. The base color of 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 opal absorbs most of the white light which impinges upon it, save for that fraction which is diffracted as glorious colors.
About text is from "A Field Guide to Australian Opals", Barrie O'Leary
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