Quartzite is a metamorphic rock formed by the metamorphism of quartz-rich sandstone. During metamorphism, quartz grains recrystallise and interlock, producing a hard, non-foliated rock. Shale metamorphoses to slate, phyllite, schist, or gneiss. Limestone metamorphoses to marble. The key protolith for quartzite is sandstone (or chert for some varieties).