Topaz is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of aluminum flurohydroxysilicate, their full chemical compound being Al2(F,OH)2SiO4.
The transparent, colored crystals, which also have good luster, are widely used as gems.
Topaz is a silicate of aluminum containing fluorine and hydroxyl which occurs in a variety of delicate colors, nicely added by impurities.
It is often found in short to long crystal prisms with pyramid-shaped ends, or just clean finished edges. It is often white, semiopaque, milky, or a faded yellow, but may be transparent, colorless, honey colored, golden brown, brown, or blue; much more rarely, pink or reddish.
Topazes richest in hydroxyl have a low density and high refractive indices, but those rich in fluorine have a higher density and low refractive indices. The stone has easy perfect basal cleavage.
Topaz normally grows in environments of pegmatitic and pneumatolytic deposits which form late in the crystallization of hot magma. So it is often found in dikes and contact aureoles around areas where granite has intruded in gaps of pre-existing rock. It is also found in alluvial deposits, but is, perhaps, the most vulnerable of the harder gemstones.
The mineral first scientifically categorized as topaz came from Saxony. The clear colorless topazes of Minas Gerais, Brazil are famous, being of gigantic proportions, and without impurities. Topazes are also found in Mexico, the USA, Sri Lanka, Japan, Russia (Siberia and the Urals), Nigeria, and Zaire.
Varieties of topaz are:
Yellow topaz -
This is the most typical variety, also called "true topaz." Its brand of yellow is easily distinguished, and different shades are sometimes referred to as "golden topaz" or "sherry topaz."
Its color can change from golden to honey yellow, or can be golden brown or honey with a pink or reddish tinge. The crystals usually receive an oval cut, but pear shapes and other elongated shape cuts are also used. A crown and pavilion is added to the cut, made up of many smaller facets shaped like lozenges. This detail brings out the luster of the stone. The longer stones often look darker at the ends. Because of its easy basal cleavage, topaz is rather brittle, so they should be treated with care and protected from the type of sharp blows which ring stones have to put up with.
Pink topaz -
The tinge of pink in the true/yellow topaz can sometimes be in a whole range of color gradations from yellow to pink, so it can be hard to distinguish between the two varieties. In some cases, the color is pink with a distinctive yellowish or orange shade, but it is more often a definite light to medium pink, tending to red or violet in deeper colored stones. The stones usually have few inclusions and are strongly transparent and lustrous.
Blue topaz -
This is the variety of topaz most readily available on the market, even though genuinely occurring blue topaz is now rare. It has a definite, uniform sky-blue color, usually without any gradiations and without inclusions. It is often pale, but it can be bright, or very rarely an intense blue. It sometimes has a slight gray or even greenish tinge, making it look a bit dull. Gemstones of several carats or even several tons of carats in weight are common.
Blue topaz at first sight may look like aquamarine, but look closer and you'll see the difference. Aquamarine always displays a very attractive pleochroism from blue to greenish blue or even bluish green. Topaz is usually a more definite unvarying blue.
Colorless topaz -
This is the most common variety of transparent topaz, although it is not as widely valued as a gemstone, compared with the colored varieties. This is because almost everyone will compare all colorless gemstones with diamond, and their lower refractive indices lesser dispersion making them far less attractive. But clear topaz has the advantage over the others of being found in large boulder-size stones and quantities, and of having good hardness and luster, and can be breathtaking in such a state. Therefore it is of some interest to hobbyists and collecotrs. Its value is one of the lowest for transparent gemstones, even though it is greatly appreciated by amateur cutters as a raw material. It is being used increasingly for the production of blue topaz by means of irradiation.
Topaz was valued as a gemstone since the times of ancient Greece, prized for its hardness, pleasant color, and luster. In earlier times, the name topaz mainly referred to a gemstone, olivine, extracted on an island then known as Topazos or St. John, and now called Zebirget in the Red Sea. It was probably also given to other, yellowish or yellow stones which in ancient times were not yet distinguished from one another, as knowledge about their chemical content and physical properties was not yet explored. The name began to be applied to the mineral and gemstone now known as topaz in the first half of the eighteenth century.
Well-colored, medium-large specimens of true topaz are quite valuable, but perhaps less so today than in the past. It is on the same level as secondary gemstones, like the better tourmalines, but its value has 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.
For pink topaz, the most common cut is the oval or pear shape, but many other old or antique faceted cuts are seen, as in the step cut. While it has always been comparatively rare, pink topaz was much appreciated in antique jewelry and stones weighing up to 10 carats are often found. When the color is fairly intense, it is one of the most valuable of the second level of gemstones, in a class with aquamarine. But specimens that are too pale have a low value. Like yellow topaz, it was perhaps more highly prized in the past than it is today.
At one time, glass imitations of pink were occasionally produced. In antique jewelry, very pale stones were sometimes given a closed setting with a painted base to heighten their color. The pink color of many topazes is due to heat treatment of many pinkish-yellow stones from Brazil. This method was used way back several centuries ago, so there's a possibility that even antique stones may have been colored in this way.
Blue topaz on the market is almost always free of inclusions because when large amounts of material were available, less clear pieces were discarded. The most common cut is oval, but all the mixed cuts, plus the step cuts are used. As with all light-colored gemstones, the value of blue topaz increases with the intensity of color, provided this is attractive and not kind of gloomy. Although blue topaz has not been manufactured synthetically on a commercial scale, a completely natural-looking blue colored stone has been produced during recent years in colorless topaz by means of exposure to radiation, or a radioactive substance.