Quartzite is a non-foliated metamorphic rock composed almost entirely of quartz. It forms when quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism.

Explanation:

In the areas of its extent where the quartz grains have been metamorphosed into a durable interlocking mass, a single rock unit such as the Tuscarora Sandstone in the central Appalachians might fully satisfy the description of quartzite. In other places, it might be better referred to as “sandstone.”