Topaz - the colorful gemstone

Publikováno: 08.11.2018 10:54

Topaz is a very popular mineral and gemstone, particularly for use in jewelry. Most nice crystals come from various pegmatite pockets around the world and are highly sought by mineral collectors. Finding topaz is quite tricky quest - and even more tricky is faceting topaz into gems.

Since the color of topaz can be changed (even colorless topaz) depending on treatments that are applied to the stone, it is used very frequently and is known for being less expensive than many other jewelry gemstones. However, a naturally occurring piece of colored topaz is likely to be considerably pricier, simply because it has not been treated. Topaz is a beautiful and complicated mineral that is normally found in colorless and yellow hues.

Crystal of pale blue topaz from locality Shengus in Pakistan
Natural pale blue topaz crystal from Shengus, Pakistan (45 x 27 mm). Photo: Vitezslav Snasel

The term "topaz" was originally given to any yellow stone, which means that even today, it can be difficult to identify a true topaz - especially a yellow one. Sometimes, yellow topaz is mislabeled as citrine.

Physical Properties/Structure

The chemical formula of topaz is Al2SiO4(F,OH)2 and it is known as a very hard mineral. It is classified as a silicate chemical with an orthorhombic crystal system. It most commonly has a vitreous luster, and is transparent to translucent, with a basal type of cleavage. On the Mohs scale of hardness, topaz registers as an 8. Although it is quite hard, topaz can still be broken easily due to its basal cleavage. This means that it is used more often in necklaces and earrings than in other types of jewelry or other items, since it can crack so easily if it happens to be struck.

Group of complex champagne and yellow colored crystals of topaz from Gorkhi-Terelch in mongolia
Various pegmatite topaz crystals up to 4 cm from Gorkhi massif, Mongolia. Photo: Zbynek Burival

Origin

Most commonly, topaz originates from felsic igneous rocks. It is a fluorosilicate, meaning that it contains flouride, silicon, aluminum, and oxygen. Although igneous rocks are the most common occurrences of topaz, they also exist within hydrothermal veins, and, very rarely, in metamorphic rocks. Topaz is generally found in granite pegmatites, where it crystallizes into very coarse crystal grains, or in certain rhyolite lava flows, which are heavy in silica concentration-particularly feldspar and quartz. Topaz often forms huge masses within greisen deposits, but only rarely forms well shaped crystals.

Macrophoto of the honey colored topaz crystal from Thomas Range in Utah, USA
Honey colored topaz crystals from rhyolites in Thomas Range, Juab Co., Utah, United States of America (FOV 7 x 7 mm). Photo: Vitezslav Snasel

Topaz varieties

One of the reasons why topaz is so popular is because it can be found in several different varieties. It exists in colors including amber, green, blue, gray, pink, yellow, white, and even colorless. Some of these types of topaz, however, do not occur in nature. Blue topaz, for example, is usually created by applying radiation to colorless topaz specimens-this is also known as being "treated." Treated topaz also appears in pink and red, as well as green. Naturally, pink and red topaz crystals do occur, but they are quite rare and very expensive. They are much more commonly found as heat-treated colorless topaz. Green topaz, when not naturally occurring, is treated by applying chromium to the outside of a colorless piece of topaz. It is important to pay close attention to a particular specimen, as treated topaz is very common. Yellow and greenish needle like topaz crystal masses from greisen veins in Germany are known as Pyknite.

Ten centimeters thick vein of topaz variety pyknite in the Altenberg mine, Germany
About 10 cm thick vein of the elongated yellow-green pyknite crystals, the center of the vein is filled with zinnwaldite mica. Old tin mine in Altenberg, Germany. Photo: Zbynek Burival

Occurence

Almost all high quality and large topaz crystals come from pegmatites in Brazil, Afghanistan, Pakistan, Russia, Ukraine, Myanmar, China and the USA. Because of its popularity amongst Czars of Russia, it is sometimes referred to as Imperial Topaz. The only two productive mines for natural golden colored imperial topaz are in Brazil. Very nice topaz crystals come from rhyolites in Thomas Range, Utah. Topaz is also very common in greisen deposits, where it rarely forms nice crystals. Famous classic greisen deposits are Schneckenstein and Altenberg in Germany or Krupka and Krasno in Czech Republic.

Macrophoto of the small clear topaz crystal from Krupka in Czech Republic
Small topaz from greisen deposit in Krupka, Czech republic (FOV 6 x 6 mm). Photo: Vitezslav Snasel

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