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!
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
Rough black opal with no fire.
