Opal has been admired for its great beauty since ancient times, but this
gemstone lacked commercial appeal until the discovery of the Australian
black opal late in the 19th century.
Opal is somewhat brittle, is sensitive to heat, and, in some cases,
tends to deteriorate despite the best of care. Therefore, this stone
lacks many of the physical characteristics required for an ideal gem.
These deficiencies would eliminate other species from the list of
gemstones, but the great beauty of its flashing and shifting color
patterns has made opal increasingly popular. Even its name, coming from
the ancient Sanskrit “upala,” means precious stone.
With a hardness between 5½ and 6½, opal is the softest of the more
popular gems. It is sufficiently hard, however, to be used in jewelry,
where its setting usually helps to protect it from shock and abrasion.
[Illustration: Black opal, so called because the color flashes
appear against a dark background, is found in Australia. It is quite
rare, and large pieces such as the ones shown here have become
extremely valuable. (Almost actual size.)]
Opal is unlike most gemstones in that its flashing color is not due to
the color of the stone itself, or even to the color of its included
impurities. Rather, it is due to the way in which tiny opal particles
are grouped during its formation. Detailed photographs taken through an
electron microscope show clearly how precious opal is deposited as
spheres that are so small they are indistinguishable under powerful
optical microscopes. These spheres are packed together in very orderly
networks, row upon row and layer upon layer, with tiny open spaces, also
in rows, between them. Masses of common opal lack this orderly internal
arrangement of spheres. White light striking the precious opal is
reflected independently by each row of spheres, much like the
reflections from a series of slats in a venetian blind. Since these rows
of spheres are spaced at distances approximately the same as the
wavelength of light, a phenomenon known as _diffraction_ occurs. The
separate reflections interfere with each other in an organized manner,
cancelling out some of the light wavelengths and reinforcing others,
producing color. The brilliant color flashes are of different hues
depending on the sizes of the spheres of opal and, therefore, the
distances between rows. To provide the best display of this optical
effect, opal is almost always cut in cabochon form rather than as
faceted stones.
[Illustration: Fire opals have rich fire; some have background
colors that vary from bright yellow through orange and red; and some
are colorless. Stones such as the ones shown here, which weigh 7,
11, and 22 carats, have made Querétaro, Mexico, famous as their
source. (Actual size.)]
[Illustration: This rare 34-carat opal from Brazil resembles closely
the opals found in Australia. (Actual size.)]
Common opal, which shows milky opalescence, does not exhibit color
flashes, and it is not used as a gemstone. Each of the common
varieties—such as hyalite, cacholong, and hydrophane—has its own
slightly different set of characteristics, but only precious opal, with
its dazzling color display, is important for gem purposes. To take full
advantage of the small amounts of gem material available, or to bring
out its color better, _precious_ opal is often cut as thin pieces and
mounted as doublets on some other backing. Also, the seams in rock
sometimes are cut so that the thin layer is exposed on a thicker backing
of the adjoining rock. Precious opal, or gem opal, is classified as
_white opal_ when the color flashes are in a whitish or light
background, _black opal_ when the background material is gray,
blue-gray, or black, and _fire opal_ when the background is more
translucent and red, reddish orange, or reddish yellow.
Precious opal has been found in several areas of the world—in nodules,
in seams in rock, or as replacements of other minerals or even of wood
and shell. Hungarian deposits were well known in Roman times, but these
and other deposits became insignificant with the discovery of opal in
Australia in the late 19th century. Opal deposits were discovered in
1889 at White Cliffs in New South Wales, and other important discoveries
in Australia followed, including deposits at Lightning Ridge in New
South Wales that produce very dark stones and the rich fields of white
opal at Coober Pedy in South Australia. Mexico has remained for a long
time the principal source of richly colored fire opals, with the most
important deposits located in the state of Querétaro, where mines have
been worked intermittently since 1835. This has made the town of
Querétaro today the center for the trade and cutting of Mexican opal.
VARIETIES
White opal: Color flashes in light-colored background material
Black opal: Color flashes in dark gray or bluish background material
Fire opal: Orange or reddish background material