Topaz

In Chinese, topaz is also known as ‘yellow gemstone’ or ‘yellow crystal’. It is an aluminium silicate containing fluorine, and its composition is Al₂[SiO₄] (F. OH)₂. The crystal structure is prismatic with striations on the faces parallel to the long dimension. It has a Mohs hardness of 8 and a specific density of 3.53–3.57 g/cm³. It is commonly yellow but can be red, pink, blue and colourless, and blue, wine yellow and pink topaz is most valuable. Its colour can be altered artificially to become azure or golden yellow. Special attention should be paid when purchasing topaz. Gem-grade topaz forms under high-temperature conditions with volatile components. It mainly exists in granite pegmatite and in placer deposits. Famous mining countries include Brazil, the United States and Russia. Topaz is also produced in Inner Mongolia, Xinjiang Province, and the Dabie Mountains in China (Fig. 25).

Fig. 25

Topaz crystal