Tectosilicate Mineral Amazonite

Associated Smithsonian Expert: Michael A. Wise, Ph.D.

Michael Wise investigates granite and pegmatite dikes that intruded into metasedimentary rocks near Mount Antero, Colorado.

Courtesy of Jennifer C. Kelly

Dr. Michael Wise is a geologist and an expert on pegmatites, which are coarse-grained igneous rocks rich in the minerals quartz and feldspar. A native of Smithfield in southeastern Virginia, Dr. Wise first got interested in mineralogy as an undergraduate at the University of Virginia, when rock-hunting trips to central Virginia and North Carolina strengthened his interest in this type of rock. He obtained his doctorate from the University of Manitoba in Canada in 1987 and joined the Smithsonian National Museum of Natural History Department of Mineral Sciences one year later. He has traveled widely in the Appalachian mountains of the eastern United States to study pegmatites, which are good sources of not only semiprecious gemstones such as aquamarine, rose quartz, and topaz, but also rare elements, such as beryllium, lithium, and cesium, with important economic applications. His research has also taken him to Nevada, Colorado, California, and the Northwest Territories of Canada. In the laboratory, he uses a scanning electron microscope, cathodoluminescence microscope, X-ray diffractometer, and other tools to investigate the composition, crystal structure, and evolution of pegmatites.

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This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.
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I love the patterns!
Magma rich in silica was injected into cracks in the older gneiss, creating this silicic intrusion, mainly consisting of pink feldspar and white quartz at its core. Location: Morrison, CO.
Photographed by Donald E. Hurlbert, Smithsonian Institution

Crystals and Pegmatites

Pegmatites are extremely coarse-grained intrusive igneous rocks, containing crystals that are both large (at least 5 cm or 2 inches across) and packed closely together. Pegmatites crystallize during the final stages of granite formation. The same silicate minerals that form granite - quartz, feldspar, and mica - generally make up the bulk of pegmatites, too, but the individual minerals in pegmatites can be many centimeters or even several meters in diameter. Some pegmatites also contain less common minerals, such as garnet, albite, lepidolite, beryl, and fluorite. Geologists and miners sometimes find beautiful, gemstone-quality crystals of topaz, beryl (aquamarine), rose quartz, smoky quartz, and other minerals within pegmatites.

The Rosser Reeves star ruby, a corundum specimen from Sri Lanka cut as a 138.7-carat oval cabochon
Photo by Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution

Gemstones and Other Ornamental Stones

Gemstones are minerals that are cut and polished to make beautiful "stones" for jewelry and other adornments. Humans traditionally consider diamonds, rubies, sapphires, and emeralds to be the four "precious" gemstones, with all other gemstones called "semiprecious." Many gems are transparent, meaning they allow some or most of the incident light to pass through them. To make a stone sparkle, gemcutters cut and polish the facets, or flat outer surfaces, of a gemstone to boost the number of times a light beam will reflect internally before leaving the stone and reaching the eye. Some semiprecious stones, such as malachite and lapis lazuli (a rock containing the mineral lazurite), are opaque to light; gemcutters polish them to a high gloss, without facets. This shiny, rounded type of gemstone is called a cabochon. Corundum (ruby or sapphire) specimens that contain tiny fibers of other minerals also may be cut into cabochons to create a luminous "star" effect on the stone's surface.