What Type of Rock Is a Crystal?
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Crystals and Minerals
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The atoms of a mineral form geometric shapes, or crystals. Minerals are defined by two things: their chemical composition and the manner in which they crystallize. So crystals are composed of minerals. Because the number of elements is finite, the number of possible crystals is also finite. Crystals are polyhedral forms (a geometric solid) with specific faces, edges and corners. These shapes are further classified in various systems, such as the isometric system, tetragonal system, monoclinic system, triclinic system, etc. Crystals may form in isolation, but more often they occur in groups or aggregates or are twinned.
Rocks and Crystals
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Rocks are naturally formed aggregates of minerals, sometimes including noncrystalline substances. As such, from mineral crystals, rocks are "grown." Igneous rocks form slowly when a body of molten rock, or magma, is crystallizing. Silicates are commonly found in igneous rocks. Sedimentary rocks, created by layered deposits of sediment, are the remains of preexisting rocks. These rocks cover two-thirds of the Earth's surface. Metamorphic rocks undergo a solid-state transformation at temperatures and pressures usually different than those the original mineral formed under. Commonly, these rocks experience recrystallization.
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Quartz
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Pure (colorless) quartz is also called rock crystal. Quartz is a hexagonal system crystal formed of silicon oxide. The crystals are generally prismatic and often resemble a pair of six-sided pyramids joined at their bases. Quartz is one of the most common minerals in the Earth's crust. It accounts for 12 percent by volume and crystallizes directly from igneous magma. It has many uses in industry, is able to rotate the plane of polarization of light, is transparent in ultraviolet rays, and is useful as a very hard abrasive. As well, some varieties (including amethyst, citrine and rose quartz) are semi-precious gems.
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