Quartzite is a metamorphic rock formed when quartz-rich sandstone or chert has been exposed to high temperatures and pressures. Such conditions fuse the quartz grains together forming a dense, hard, equigranular rock. The name quartzite implies not only a high degree of induration (hardness), but also high quartz content. Quartzite generally comprises greater than 90% percent quartz, whereas the sandstone will break around quartz grains. Quartzite also tends to have a sugary appearance and glassy luster.