Quartzite is a metamorphic rock consisting largely or entirely of quartz. In the vast majority of cases, it is a metamorphosed sandstone.The transition from sandstone to quartzite is gradational. There is little mineralogical change. Quartz, the main constituent of sandstone, is not altered to other minerals if the sandstone is relatively pure, but it is recrystallized during the metamorphism. Original grainy look of sandstone is lost to a variable degree. Some quartzites are still similar to sandstones, only more strongly held together, while others are completely recrystallized so that all the features like fossils or original texture are obliterated and grain boundaries have disappeared. It is difficult to say where the boundary between these rocks exactly is. Rock sample that may be quartzite for one geologist might be strongly cemented sandstone for another.
