Sandstone turns into a hard metamorphic rock called quartzite. Quartzite is formed when sandstone goes through metamorphosis under intense heat and pressure. Quartzite is mostly white or gray, and it breaks in even layers to reveal glassy surfaces with visible sand and quartz grains.
Quartzite is mined in states such as New York, Pennsylvania, Idaho and South Dakota. Because of its hardness and density, quartzite accounts for only 6 percent of all crushed rocks processed in the United States on an annual basis. Its attractive quartz pattern does, however, make it a popular dimension stone for walls, floors, stair steps and roof tiles.