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A gemstone that is generally yellow, ranging from canary-yellow to orange-yellow, but sometimes is colourless or of a wide range of other hues, including pale blue, pale green, pink, golden-brown, and sherry-brown. It is very hard but has strong cleavage and breaks easily; it has double refraction, low dichroism and colour dispersion, and a vitreous lustre, and is pyroelectric.
The topaz is often cut as a pendeloque, but sometimes mounted as a mixed cut and some stones (especially when colourless) are brilliant cut. The topaz is resembled by some other gemstones, e.g. the colourless variety, by diamond, rock crystal, and white corundum; the yellow variety, by yellow sapphire (sometimes miscalled 'Oriental topaz') and yellow quartz (which, when heated, is citrine, and sometimes called 'false topaz', 'Brazilian topaz' or 'Spanish topaz'); the greenish-blue variety, by aquamarine, and the pink variety, by tourmaline (especially pink rubellite).
Some yellow topazes change colour by heat treatment, e.g. the rose topaz; but quick heating can remove all colour. There is no commercial synthetic topaz, but the term is sometimes applied to a coloured synthetic corundum. Among the local misnomers that have been applied to topaz is 'Brazilian ruby'.
The name 'topaz' was for centuries applied to a stone which was found on the Arabian Gulf Island of Topazos.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson