topaz（黄晶，黄玉） is a hard,
1 mineral. it is a compound of
2, silica, and fluorine（氟）.
3 topaz is valuable. jewelers call this variety of the stone "precious topaz". the best-known precious topaz
4 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
5.
the golden variety of precious topaz is quite
6. most of the world's topaz is white or blue. the white and blue crystals of topaz are large, often weighing thousands of
carats（克拉）. 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
7 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（海蓝宝石） and a variety of brown quartz is widely sold as topaz. the quartz is much less brilliant and more
8 than true topaz. most of it is a variety of
9（紫水晶）: that heat has turned brown.
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