Quartzite

Hardness of 7 on the Mohs scale; typically white, gray, or pinkish; vitreous to greasy luster; non-foliated crystal structure; conchoidal fracture; specific gravity around 2.65.

Formed when quartz-rich sandstone or chert is subjected to high heat and pressure, typically through tectonic compression in orogenic belts. Geological age varies widely from Precambrian to Cenozoic.

Used extensively in construction as crushed stone for roadbeds, railway ballast, building facades, and decorative landscaping. Also used to manufacture silica brick and in metallurgical processes.

Quartzite is extremely durable and resistant to chemical weathering, which is why ridges formed of quartzite often persist as prominent mountain crests. Unlike sandstone, quartzite fractures across the grains rather than around them.

Can be identified in the field by its hardness (it easily scratches glass and cannot be scratched by a steel knife) and the sugary texture of interlocking quartz grains on freshly broken surfaces. Common worldwide, particularly in mountain ranges and ancient cratons.