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White precious opal
This is the variety which has been known and
used for the longest time, and was, up to the end of the nineteenth
century, the noble opal par excellence, praised by Pliny the
Elder as the ultimate in gemstones, due to its marvelous and
mysterious, iridescent qualities.
Appearance It has a whitish (watered-down
milk) to light grayish, dull yellow, light blue-gray, or pale-blue
grounc' color. The range of colors of the patches, due to
diffraction, depends on the size of the minute spheres of
which the gem is composed: patches will be violet to blue
for structures with very small spheres, gradually turning
tc green, yellow, orange, and red as the size of the spheres
increases. The wavelengths diffracted depends on the distance
between rows of spheres. When this becomes toc great, diffraction
no longer occurs. The patches of color can be more or less
clearly defined, extensive, and homogeneous in size. Pieces
with angular, polygonal, evenly distributed patches in a wide
range of color with clear-cuedges are known as harlequin opals:
this is the most valuable variety, for both black and white
opal. Varieties witF heterogeneous patches of color and poorly
defined edges are less highly prized.
White opal is, whenever possible, cut into fairly convex cabochons.
It is sometimes cut flat but here there is a risk c•
breakage, as the stone is brittle. The most valuable cabochons
are the strongly curved, oval ones. Many otherhave a vaguely
triangular shape with rounded corners or at any rate, less
symmetrical shapes, which permit a highe, yield from irregular
rough stones.
Distinctive features White opal is unmistakable, mor~immediately
recognizable than any other gem. There is, c' course, the
problem of distinguishing it from black opa given the wide
range of gradations from one to the other i is called black
when the background color is mid-gra> smoke gray, blue
or black; otherwise it is called white.
Occurrence As already mentioned,
white opal has bee mined for centuries in Czechoslovakia;
but productio there is nowadays extremely limited. Most white
opal no.. comes from Australia, mainly from the Andemooka
ar Coober Pedy deposits in South Australia. Light-colore opal,
including some fine opal, with the iridescence
noble opal, also comes from Mexico.Smaller quantities are
also extracted in the United State Brazil, Japan, and Indonesia.
Value The best quality gems fetch very high
prices, e• ceeded only by the four principal gemstones
(diamor, emerald, ruby, and sapphire), imperial jade, alexandrit
and black opal. The potential price range, however, is ve,
extensive and hard to quantify without direct experiencMany
specimens which are very pleasing in appearaf are quite modestly
priced, costing no more than other s ondary gems. Specimens
with weak, barely visible irid,cence
are fairly cheap.
Simulants and synthetics It was long considered
possible to imitate opal. In recent years, however, imition,
which, at first sight, looks deceptively similar to o; has
appeared on the market. It is called ' Slocum stonF after
its inventor. A plastic imitation has also recently cc onto
the market. It is very similar indeed to the natw stone, being
composed of microscopic spheres, like or Fortunately, its
low melting point, hardness, and dens can distinguish it,
despite its appearance. White opal h been manufactured synthetically
by a French company nearly a decade; the appearance of its
iridescent patcr is fairly distinctive (each being in the
form of a mosaic), r it can nonetheless only be distinguished
by an expert.
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