OPAL
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.
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.
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.)
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