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Black precious opal
This variety has only been known and appreciated
sin the beginning of the twentieth century, after the discova
of the Australian deposits. Before that, it was only fou occasionally,
as a true rarity, and was regarded with sormistrust. In some
cases it was known to be obtained by tificial coloration of
the mass, using the same procedur used for agate, to highlight
the patches of iridescence.
Appearance It has a bluish
gray, smoke gray to bla color. This dark ground greatly enhances
its appearan by emphasizing the patches of color caused by
diffractic The range of colors and shapes of the patches are
rougt the same as for white opal, and the standards and termin
ogy employed are identical. Here, too, when the patcr are
angular, polygonal, clear-cut, and of uniform size, ia called
(black) harlequin opal, and this is the most valuat type of
all. Naturally, the cabochon cut is used and thicker gems
are preferred, those of regular outline, better suited to
jewelbeing the most highly prized.
Distinctive features Like white opal, it
is unmistakab, Only the dividing line between black and white
opal is r clear-cut, one type merging into the other. Care
needs be exercised to ensure that what may appear to be a
bla - opal is not really a thin layer of white opal on a piece
dark-colored rock. Examination of the stone from the x will
reveal this combination.
Occurrence Black or dark opal comes almost
exc sively from Australia, the main deposit being at Lightn
Ridge (New South Wales); less important sources are T tenbar
(New South Wales) and Mintable (South Australia Very small
quantities also come from Indonesia.
Value Very high; the highest for opal, and
therefore imediately after the principal gemstones (diamond,
emaid, ruby, and sapphire). Naturally, the value is sligt
lower when the contrast between the iridescent patch and background
color is less pronounced, when v patches are less clearly
defined, and when the range colors is limited; or, finally,
when the patches are arranged haphazardly, are uneven, or
at any rate less attractive Nevertheless, all black opal is
expensive.
Simulants and synthetics Black opal is harder
to imitatF than white opal. The French company which produces
synthetic white opal has succeeded in producing very attrac
five black opal, but at a very high cost, which few are willing
to pay for a synthetic stone because, for the same price,
one could obtain quite a fine, natural white opal. Because
of the value of black opal, even very thin veins arc used
for ornamental purposes.
By reason of opal's extreme brittleness, it
is used either as the base of doublet, with the top, convex
part of colorless quartz, or as an irtermediate layer of triplets,
the base of which consists c common opal and the top part
of quartz. These doublet and triplets are not regarded as
false, but are obviousl. much less valuable than all-opal
cabochons. But when, and sometimes happens, the triplet is
made with white opal, th, underside of which is cemented to
the bottom layer witr black glue, to make the entire gem look
more valuable, this is a clear case of falsification. Recently,
treatment of whit, opals has recommenced, taking advantage
of their natural. porosity to darken their color, and thereby
increase the value. These processes are very hard to detect.
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