Topaz is a common gem stone. Topaz crystals can reach very large sizes, with crystals in pegmatites occasionally measuring several metres long and weighing several hundred kilograms.
Topaz occurs mainly in felsic igneous rocks such as granite, granite pegmatite and rhyolite, and is often found in veins and cavities in such rocks.
Chemical composition - Al 2{SiO 4}(OH, F) 2
Hardness - 8
Specific gravity - 3.5-3.6
Transparency - Transparent to translucent
Colour - Colourless, pale yellow to amber; also pale shades of blue, green, orange, red
Streak - White
Lustre - Vitreous
Cleavage/fracture - Perfect in one direction / conchoidal
Crystal habit/mode of occurrence - Prismatic (with a variety of terminal pyramids and pinacoids) / massive, granular.
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