Discovering Gemstones - November Edition
There was a time when any yellow gemstone would be called topaz. Today, the word topaz refers to a silicate mineral that contains aluminum and fluorine, Al2SiO4(F,OH)2. With a hardness of 8 topaz is the third hardest mineral on the Mohs Hardness Scale and an excellent mineral for making gemstones.
Topaz comes in a variety of colours, from clear to oranges, pinks to purples. Most people think of topaz as being blue, but a blue topaz is actually rare! In most cases a blue topaz is actually a clear topaz that has been treated with radiation and high temperature to change its colour. Natural colours in topaz are caused by two factors: tiny imperfections in the crystal structure create yellows, oranges, browns and blue, while chromium impurities in the gem will give it a pink to purple colour. Topaz can also be pleochroic, which means that it will appear to be different colours in different lighting conditions and when viewed from different angles.
Topaz occurs in pegmatite, a felsic igneous rock with crystals that range in size from 2.5 centimetres to several metres wide. Because of this many of the world’s largest faceted gems are topaz, the largest in the world being the “El-Dorado Topaz” weighing in at 31,000 carats or 6.2kg it was discovered in Minas Gerais in Brazil.