A 3273-carat topaz of soft blue that came from Brazil. The Smithsonian Institution had this unique gem cut by Capt. John Sinkankas of California. For several years it was the largest topaz in the collection. (Both gems are shown in actual size.)
Topaz, an aluminum fluosilicate, has a hardness of 8, a vitreous luster, and a relatively high refractive index. It is found in near-perfect crystals that range in size from very small to very large, with some giants weighing as much as several hundred pounds. Most of these crystals, especially the largest ones, are colorless, a characteristic that indicates relatively high purity of composition. Although topaz gems have little fire, they take a high polish and can be very brilliant. Great care must be taken in cutting and polishing topaz because of its ready cleavage. The desired cut and high polish can be secured by avoiding excessive heat or pressure during the operation and by planning facets so that none lies exactly parallel to the cleavage direction.
Although crystals of gem-quality topaz are found in many localities, perhaps the splendid blue ones from Russia and the yellow, wine, blue, and colorless ones from Brazil are best known. Some fine topaz has been found in the United States in such widely separated areas as New Hampshire, Texas, Colorado, and California. The light, golden brown topaz from Colorado has an unfortunate tendency to fade in strong sunlight. It remains to be seen whether similar topaz coming recently from comparable occurrences in Mexico also will fade. By a system of heating and cooling, certain of the red-brown topaz crystals from Ouro Preto, Brazil, can be converted to colors ranging from salmon pink to purple red. Quick heating to high temperatures can completely remove color, and sudden or uneven cooling may cloud or crack the stone.
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.)