Quartzite is a metamorphic rock formed when quartz-rich sandstone or chert has been exposed to high temperatures and pressures usually related to tectonic compression within orogenic belts. 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, and some examples, containing up to 99% quartz, and are the largest and purest concentrations of silica in the Earth’s crust.
Although quartz-rich sandstone can look similar to quartzite, a fresh broken surface of quartzite will show breakage across quartz grains, whereas the sandstone will break around quartz grains. Quartzite also tends to have a sugary appearance and glassy luster. The variety of colors displayed by quartzite is a consequence of minor amounts of impurities being incorporated with the quartz during metamorphism. Although quartzite can sometimes appear superficially similar to marble, quartzite will not be scratched by a metal blade, and will not fizz on contact with dilute hydrochloric acid.
Quartzite is granular, medium grained and hard. Generally it feels gritty to touch. Pure quartzite is white but quartzite exists in a wide variety of colors.
As a building stone, Quartzite is used as a dimension stone for building facings and paving.
Quartzite was used on the Graham County Courthouse in Robbinsville, NC.