Quartzite is a type of metamorphic rock without noticeable foliation (the arrangement of structural or textural features in any rock). Quartzite is composed mainly of quartz; the rock type usually forms by recrystallization of sandstone or chert under conditions of regional or contact metamorphism, which is caused by changes in chemical and physical conditions, such as heat or pressure.
Quartzite is generally buff to pale brown, but it can be almost any colour, depending on the chemical impurities present. Samples often display a sugary appearance on a freshly broken surface. Metamorphosed quartzite can be distinguished from silica-cemented sandstone by careful inspection of a fresh surface: In sandstone, the rock breaks around mineral grains, while in quartzite, the rock breaks through the grains.
Quartzite's properties are similar to those of the mineral quartz. A sample of quartzite has a density of approximately 2.3 to 2.4g/cc. The rock type's hardness of approximately 7 on the Mohs hardness scale makes it very resistant to weathering. Porosity is generally low to very low.
The word "quartzite" has been applied to both the quartz-rich metamorphic rock and to unmetamorphosed hard, silica-cemented sandstone comprising nearly pure quartz grains. Use of the term for a sedimentary rock is discouraged in favour of "orthoquartzite" to reduce confusion.