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Topaz |
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Topaz is a silicate mineral most often found in igneous rocks of felsic
composition. It is also found in some hydrothermal veins and
the hydrothermally altered rocks that surround them. It
is a common mineral of pegmatites and also found in the cavities
of rhyolite and granite. Rarely, topaz is found in metamorphic rocks
such as schist or quartzite. Topaz is not an abundant mineral but
occurs worldwide in the types of rocks listed above.
Compared to other minerals, topaz is relatively resistant to weathering and stream abrasion. These characteristics enable it to be found in the stream sediments derived from the weathering of topaz-bearing rocks. It is sometimes produced by placer mining. Topaz GemstonesTopaz is a very popular gemstone. Gem quality topaz most commonly occurs in nature as a colorless crystal. When cut as a gemstone these colorless specimens generally have the lowest value. A significant amount of natural topaz also occurs in a color range spanning from yellow to brown. Small amounts of natural blue and pink topaz are found but the abundance of natural stones in these colors is very low.
Treated Topaz GemstonesBlue topaz is a very popular gemstone but this color is rarely found in nature. To produce blue topaz, some dealers irradiate clear topaz then heat it to produce a blue gemstone. The most desirable blue topaz stones have a color that resembles aquamarine. Today, most of the blue topaz being sold as a gemstone has been treated. Dealers should label these stones as "treated" or "enhanced" so that customers know the stone's history. The price of blue topaz has declined in recent years because so much blue topaz has been produced through treatment. A small amount of topaz occurs naturally as pink and red crystals. These are the most expensive colors of natural topaz. However, brown topaz can sometimes be heat treated to produce a pink stone. Two other colors of topaz are seen in the gemstone market. One is a green topaz that has been produced by diffusing chromium into the outer surface of the stone. Another treated stone is sold under the name "mystic" topaz. This is a stone with an iridescent appearance that is produced by vapor coating the surface of clear topaz with titanium oxide. This surface coating is not permanent and many mystic topaz gemstones quickly show signs of wear. Synthetic Topaz GemstonesSome synthetic topaz is produced, cut and sold as gemstones. It should be clearly labeled as "laboratory", "synthetic", "created" or some other term to let the buyer know that it is not natural material. Anyone purchasing gemstones should be aware that treated and synthetic gems exist and are sometimes sold without proper labels. Topaz Hardness and DurabilityTopaz is a very hard mineral. It is used for the standard hardness of eight in the Mohs Hardness Scale. Some minerals of great hardness can be broken rather easily. Topaz has a perfect basal cleavage making it vulnerable to breakage. This cleavage makes topaz a somewhat fragile gemstone. It is best suited for pendants and earrings that receive little hard use and less suited as a ring stone that might be bumped while in use. |
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Topaz - Mineral Properties and Uses |
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Uses |
The only significant commercial use of topaz is as a gemstone. It also serves as the Mohs Hardness Scale standard for a hardness of eight. |
Color |
Colorless, white, yellow, amber, pink, blue, green, gray |
Streak |
Colorless - harder than the streak plate |
Luster |
Normally vitreous |
Diaphaneity |
Transparent to translucent |
Cleavage |
Perfect basal cleavage |
Hardness |
8 |
Specific Gravity |
3.4 - 3.6 |
Distinguishing |
Hardness, prismatic crystals, sometimes striated, cleavage, specific gravity. |
Crystal System |
Orthorhombic |
Chemical Classification |
Silicate - Nesosilicate |
Chemical Composition |
Al2SiO4(F,OH)2 |
Blue faceted topaz. 1.40 carats, 9x6 mm, from Brazil.
Rough brown topaz crystal sections.
Faceted pink "treated" topaz. 2.4 carats, 9x7mm, from Africa.
Faceted clear topaz. 6.25 carats, 14x10 mm, from Switzerland.
Synthetic blue faceted topaz. 12.05 carats, 14x11 mm, Russian made.
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