[Illustration: Three different cutting styles and colors of topaz.
From top, a 235-carat colorless stone from Colorado, a 171-carat
dark champagne-colored stone from Madagascar, and a 129-carat
sherry-colored stone from Brazil. (Slightly less than actual size.)]
Because yellow is the most popular color of topaz it has become
customary to believe that all topaz is yellow. Also, there is a tendency
to believe that all yellow gemstones are topaz. Neither belief is
correct. Stones of yellow, sherry, blue, pink, and colorless topaz all
make beautiful gems, and their characteristics are identical except for
color. On the other hand, citrine (a yellow quartz), although entirely
unrelated to topaz, often is disguised in the trade under the names
Brazilian topaz, topaz quartz, or just topaz. Great numbers of stones
described and sold as yellow topaz really are the much commoner citrine,
which has few of the characteristics of fine topaz.
[Illustration: A cushion-cut topaz from Brazil that weighs 1469
carats. It is an odd shade of yellow-green.]
[Illustration: 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.