The Mineral Topaz
The word Topaz was used by Greeks and Romans to describe a green precious stone as the color of fresh oil or the juice of leeks before the Middle Ages. The gems were discovered on a famous island called Topazios which is presumed today to be Zabargad in the Red Sea. During the Middle Ages the name topaz was used to describe one that was yellow or gold instead of describing a green. The reason for the change is not known. Actually real topaz is not found on the island of Zabargad since Zabargad is made up of mafic and ultramafic rocks. Topaz is found only in granitic rocks. It is commonly found along with tin ore. The type of granitic rocks that hold tin ore and topaz are called A type granite Topaz is sometimes mistaken with quartz. It is most common in its colorless form. Topaz is formed under high temperatures.
It is known to be high in Fluorine and low in water. Topaz is likewise found in granite together with minerals such cassiterite or tin oxide, muscovite and quartz. In the science of gemology, topaz is known as one of the most popular and widely used gemstone. Topaz is usually found in the form of large, well developed crystals. Some topaz samples weigh up to several hundred pounds. There are gems that have been cut from huge specimens like the 144,000 carat golden brown topaz which was exhibited at the 1974 National Gem and Mineral show in Lincoln, Nebraska and a 36,853 carat champagne topaz that was carved in 1989. Brazil is the country known to have large topaz deposits.
As one of the hardest minerals in existence and as the hardest silicate mineral, topaz is known to have a hardness of eight. Despite this feature, topaz has perfect basal cleavage in one direction making it hard to cut and polish. For this reason the carvings made out of topaz are very rare. The beauty of this gemstone is best examined closely with the use of the ore microscope. Topaz gemstone has a high luster. Because of this good crystal are formed with many facets, and good color. Under the ore microscope, topaz comes in various colors ranging from colorless to red, pink, yellow, orange, brown, and pale blue. The colorless variety is fairly common. It can be cut and polished to look like diamond. Topaz can likewise be heat-irradiated to turn it into blue. The blue topaz has become a very popular and less expensive replacement for aquamarine. Considered to be the most valuable form is the orange-yellow variety called imperial topaz.
What differentiate topaz from other minerals are its hardness, cleavage, low birefringence, positive optic sign and moderate 2V angle. Likewise, it shows high relief meaning its edges stand out distinctly from the surroundings when it is viewed under an ore microscope. It can be pleochroic, since it changes color when it is rotated under the microscope. Another distinguishing characteristic of topaz is its cleavage trace which parallel to the fast ray. When viewed under the microscope, minerals will have dark straight lines on them going in the direction of their cleavage. These lines are referred to as the cleavage trace. As the light enters any mineral, it is split into two rays where one travels faster than another. The direction of the fast ray is known as an optical property of a mineral. In the case of topaz, this ray travels parallel to the cleavage trace which distinguishes it from similar minerals like andalusite, melilite, vesuvianite, quartz and feldspars. Read the entire article
