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.

---

quartzite

Sandstone that has been converted into a solid quartz rock. Quartzites are usually white; they fracture smoothly and break up into rubble under frost action. Sandstone may be converted to quartzite by precipitation of silica from waters below the Earth's surface; such rocks are called orthoquartzites, whereas those produced by recrystallization (metamorphism) are metaquartzites. Because they weather slowly, they tend to project as hills or mountain masses. Many prominent ridges in the Appalachian Mountains are composed of quartzite. Pure quartzites are a source of silica for metallurgical purposes and for the manufacture of silica brick. Quartzite is also quarried for paving and roofing materials.