Topaz Crystalline Mineral Formed During Volcanic Cooling

​One of the highlights at the Houston Museum of Natural Science’s Cullen Hall of Gems and Minerals is a 3,000-carat topaz crystal. Topaz is a crystalline mineral within the silicate group with fluorine within its chemical makeup. Topaz forms as igneous rock cools when volatile gaseous elements such as hydroxyl group (OH) and fluorine emit during volcanic degassing and later magma cooling stages.

Forming within cavities and fractures of rocks that have already cooled, such as granite, rhyolite, and pegmatite, topaz crystals expand as hot hydrothermal fluids flow through the rock cracks. In Australia, professionals have found the mineral in granite-derived placer deposits or gravel carried by streams.

Topaz has been revered since ancient times, with Ancient Egyptians associating the golden color of the gemstone with Ra, the Sun God, and ancient Greeks associating it with strength. With topaz mined during the Middle Ages in Germany’s Erzgebirge Mountains, the mining epicenter shifted to Brazil’s Minas Gerais region in the 1730s, and the industry there remains significant. Beyond its use for jewelry and ornaments, topaz serves as the basis of refractory materials that hold their strength under elevated temperatures and are thus useful for industrial applications in furnaces, incinerators, and reactors.

Joel Bartsch

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