The metamorphic rock composed almost entirely of quartz. Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism..Hand Sample Image: .Geology - rocks and minerals. 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..Quartzites form when quartz sandstones are metamorphosed. Quartzites often preserve original cross-bedding in the sandstone; sometimes deformation during metamorphism makes this cross-bedding look somewhat 'stirred '. The way to tell a quartz sandstone from a quartzite is when you break a quartzite, the fracture will .