Silicate of aluminium containing fluorine and hydroxyl. In earlier times, the name topaz mainly referred to a gemstone, olivine, extracted on an island then known as Topaz os or St. John, and now called Zebirget in the Red Sea. It was probably also given to other, yellowish or yellow stones which were indistinguishable from one another without an adequate knowledge of chemistry and mineralogy. The name began to be applied to the mineral and gemstone now known as topaz in the first half of the eighteenth century.
Crystal system Orthorhombic.
Appearance Often found in short to long prismatic crystals, sometimes with characteristic terminal faces. lt is often white, semiopaque, milky, or tinged a dirty yellow, but may be transparent, colorless, honey colored, golden brown, brown‘, or blue; much more rarely, pink or reddish.The transparent, colored crystals, which also have good luster, are widely used as gems.
Physical properties It has a hardness of 8 and a density of 3.52-3.56 g/cma. The refractive indices are roughly na 1.610, ny 1.618 or na 1.630, ny 1.638. The density and refractive indices are interrelated and depend on the composition of the specimen: topazes richest in hydroxyl (OH) have a low density and high refractive indices, whereas those richest in fluorine have a higher density and low re fractive indices. The stone has easy perfect basal cleavage.
Genesis Topaz is characteristic of pegmatitic and pneumatolytic deposits and is therefore found in dykes and contact aureoles around granitic intrusions. It is also found in alluvial deposits, but is, perhaps, the most vulnerable of the harder gemstones to attack by the elements, at least in tropical or equatorial regions.
Occurrence The mineral first classified scientifically as topaz came from Saxony. The topazes of Brazil are famous, some being of gigantic proportions. Topazes are also found in Mexico, the United States, Sri Lanka, Japan, the Soviet Union (both in Siberia and in the Urals), Nigeria, and Zaire, to mention just the main sources.
True topaz or yellow topaz is the most typical variety, and no adjectives are needed to describe its color, although different shades are sometimes referred to as “golden topaz" or “sherry topaz."
Appearance The color can range from golden to honey yellow or can be golden brown or honey with a pink or reddish tinge. The crystals are usually out into oval gems but pear and other mainly elongated shapes are also produced, usually with a crown and pavilion consisting of very many small lozenge-shaped facets, which bring out the luster of the stone. The longer stones often look darker at the ends. Easy basal cleavage makes these gems rather brittle, so they should be treated with care and protected from the type of sharp blows to which ring stones are susceptible. Medium-Iarge stones are relatively common, and even very large ones are not rare.
Distinctive features A great deal of citrine quartz, which looks fairly similar, is sold under the name of topaz or topaz citrine, creating some confusion on the market. The color of topaz is, however, generally much warmer and more likely to have an orange or pinkish tone. The stone will also have a much greater luster. In any event, a quick check of the density, perhaps using one of the heavy liquids, would immediately distinguish between the two, and measurement of the refractive indices would remove all trace of doubt
Occurrence Topaz comes mainly from jgzil, and in smaller quantities, from the Soviet Union, Japan, SriLanka, Burma, and the United States. It is occasionally found in Germany.
Value Well colored, medium-large specimens of true topaz are quite valuable, but perhaps less so today than in the past. Roughly on a par with secondary gemstones, such as the better tourmalines, it has probably suffered from the ready availability of citrine quartz, an inferior stone, which, by usurping the name, has made topaz seem more abundant than it really is.
Simulants and synthetics The main problem is the practice of-passing citrine quartz off as topaz. Synthetic corundum of a color similar to topaz has also been produced by way of imitation, whereas topaz itself has only been synthesized on a limited scale for scientific purposes and is not found on the market.