Metamorphic Rock Garnet Mica Schist

Associated Smithsonian Expert: Richard Wunderman, Ph.D.

Dr. Richard Wunderman (right) with his graduate school adviser, Professor Bill Rose, at Michigan Technological University in 2013

Photo by Fraser Goff, Geologist

Dr. Richard Wunderman is a volcanologist in the Smithsonian’s Global Volcanism Program (GVP) at the Smithsonian National Museum of Natural History. Born in New York City, Wunderman grew up in Mountain View, California, part of the region dubbed “Silicon Valley.” He admits he was not a stellar student when he was young, and his mother encouraged him to find a career that would allow him to spend time outdoors. He and his family traveled extensively. “We had adventures in every sense of the word,” he says. In community college he was inspired by his geology, paleontology, and chemistry professors, and he later studied geology at the University of California at Berkeley. In graduate school at Michigan Technological University, he wrote his master’s thesis about a volcano in Guatemala and eventually earned his doctorate in 1988. Wunderman came to the Smithsonian in 1993, after working as a physics instructor at Front Range Community College in Colorado. As managing editor of the Bulletin of the Global Volcanism Network, he writes about active volcanoes and edits incoming reports from scientific colleagues all around the world.

Meet our associated expert

This image was obtained from the Smithsonian Institution. The image or its contents may be protected by international copyright laws.

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Gneiss from the Northern Territories, Canada
Photo by the Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences

Grading Metamorphic Rocks

Geologists use the phrase metamorphic grade to describe the degree of metamorphism a rock has experienced. The higher the temperature and pressure the rock has experienced over time, the more its metamorphic grade increases. For example, metamorphism can change mudstone, originally a fine-grained sedimentary rock, into smooth-textured slate, which splits easily into thin sheets. Over time, the slate will become phyllite, a metamorphic rock that breaks into thick slabs. If metamorphism continues, phyllite turns into schist, which is considered a medium-grade metamorphic rock, and eventually into gneiss (pronounced "nice"), a high-grade metamorphic rock. Once the rock starts to melt under still higher temperatures, it becomes a migmatite, and when it melts completely, it turns into magma which ultimately hardens into igneous rock.

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.