quartzite

quartzite, usually metamorphic rock composed of firmly cemented quartz grains. Most often it is white, light gray, yellowish, or light brown, but is sometimes colored blue, green, purple, or black by included minerals. It results from the metamorphism of pure quartz sandstone. It is most easily distinguished from sandstone by the fact that it fractures across its constituent grains of sand, while sandstone fractures along the line of the cementing material between the grains of sand. Although most quartzites are metamorphic, some are sedimentary in origin, resulting from cementation of quartz sandstone by groundwater solutions containing pure quartz.

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