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TOEFL Directory > TOEFL writing > 38 Topaz
38 Topaz
Topaz is a hard, transparent mineral. It is a compound of aluminum,
silica, and fluorine. Gem topaz is valuable. Jewelers call this variety
of the stone "precious topaz? The best-known precious topaz gems
range in color from rich yellow to light brown or pinkish red. Topaz is
one of the hardest gem minerals. In the mineral table of hardness, it
has a rating of 8, which means that a knife cannot cut it, and that
topaz will scratch quartz.
The golden variety of precious topaz is quite uncommon. Most of the
world's topaz is white or blue. The white and blue crystals of topaz
are large, often weighing thousands of carats. For this reason, the
value of topaz does not depend so much on its size as it does with
diamonds and many other precious stones, where the value increases about
four times with each doubling of weight. The value of a topaz is largely
determined by its quality. But color is also important: blue topaz, for
instance, is often irradiated to deepen and improve its color.
Blue topaz is often sold as aquamarine and a variety of brown quartz is
widely sold as topaz. The quartz is much less brilliant and more
plentiful than true topaz. Most of it is variety of amethyst: that heat
has turned brown.
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