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quartz
Photo Credit: United States Geological Survey and the Mineral Information Institute.

quartz:

A very hard mineral commonly found in sand and in many kinds of rock, such as granite, and sandstone. Quartz can vary in color and appearance from white and opaque to colorless and transparent. In the crystalline form, called rock crystal, quartz crystals are six sided and come to a point. Agate, jasper, onyx, and amethyst are different forms of quartz.