Smoky Quartz with Topaz ( #9)

A lustrous dark cathedral-growth smoky quartz with scattered blue to green topaz xtls. to 1 ¼", very different.
North America Zapot mine, Fitting District, near Hawthorne, Mineral Co., Mineral Co., Nevada, USA 9" x 5 ¼" x 4 ¼"

 

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The variety of quartz called smoky quartz ranges in color from dark brown to essentially black. We began considering the causes of color in minerals with specimen 4, which is another variety of quartz, amethyst. The cause of color in smoky quartz is, as in amethyst, a color center, although the culprit is not iron (Fe), but aluminum {Al). Pure quartz is Si02. It is not unusual for Al3• ions to replace Si .. ions in quartz in very small amounts. This creates a local charge imbalance, which favors an 02- ion being transformed into and 0 – ion by loss of an electron. This unpaired electron becomes a color center, although the process is somewhat more complex and seems to depend on subjecting the crystal to natural radiation. The smoky color can be caused in a laboratory setting by subjecting a colorless quartz crystal to intense radiation for a short time. In nature, smoky quartz occurs in granitic rocks, which naturally contain some radioactive elements. This low level of radiation acting over many years accomplishes the same color change done rapidly in the laboratory.

Because the smoky color tends to be quite uniform throughout a crystal, smoky quartz is used in making attractive jewelry. It is also reported that thin slabs of this mineral were used as sunglasses in China in the 12th century.