Quartzite

Hardness: 7 (Mohs scale), Color: Pinkish-brown or tan, Luster: Vitreous to dull, Structure: Granoblastic (interlocking quartz grains), Fracture: Conchoidal to irregular

Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together, creating a much harder rock.

Used in construction for road ballast, flooring, and wall coverings. High-purity quartzite is used to produce glass and silicon metal. It is also collected for its color and durability.

Unlike sandstone, which breaks around grains, quartzite breaks through the grains due to the intense fusion during metamorphism. It is one of the most durable and weather-resistant rocks on the Earth's surface.

Identifiable by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. Often found in mountain ranges or areas of continental uplift. In the field, look for a sugary or glassy texture on fresh surfaces.