Topaz

Topazes are usually very well-formed crystals, some of which have a large surface area, with a glass-like luster on the surfaces. Topaz is 8 on the Mohs scale and very brittle, i.e. easy to cleave with smooth fracture surfaces. Because of its good cleavage, topaz is difficult to work with; rapid temperature changes or improper setting can easily cause cracks and fissures. In its pure form it is colorless and translucent, through multiple light refraction it can appear as white due to lattice construction defects. It is often found in bright colors, especially yellow. Other naturally occurring colors include pinkish red, brownish red, violet, light blue, and light green. The coloration is caused by oxidation states of iron or chromium. Topaz is a popular gemstone that is not too expensive, because it is common. Rare color varieties are especially sought after and more valuable: deep blue stones rarely occur in nature, but even rarer are orange-red or yellow-orange stones called "Imperial topaz". Like other gemstones, topaz is treated by irradiation and heating; irradiation with gamma or electron rays results in blue to greenish-blue stones; heating produces blue and reddish color variations. Topaz can form very large crystals, the largest known have reached a length of over one meter and weigh up to 2.5 tons. Crystals weighing 100 kg and more are not uncommon. Topaz grows in different varieties, on a base as well as in massive and granular varieties. Topaz often occurs together with beryl or minerals of the tourmaline group in acid magmatic rocks, as well as in acid volcanic rocks. It also found in mineral soap in river sediments. A historically important locality was the so-called Schneckenstein in Saxony, where topaz was mined especially in the 18th century. Nowadays important deposits are located mainly in Brazil, in some of which especially large crystals have been found. Other deposits are situated in Afghanistan, Pakistan, Algeria, Burma, Japan, Sweden, Norway and the US.