Quartzite

Hardness: 7 (Mohs), Color: Pinkish-tan or light brown, Luster: Vitreous to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: Indistinct (conchoidal fracture), Specific Gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure causes recrystallization of the quartz grains, locking them into a dense, crystalline structure. Typically Precambrian to Paleozoic in age.

Used as crushed stone for road construction, railway ballast, and in the manufacturing of glass and ceramics. Polished varieties are used for floor tiles and decorative stonework.

Quartzite is so tough that it often outlasts the surrounding rocks during erosion, frequently forming the tops of ridges and mountains. It is more chemically resistant than nearly any other common rock.

Identify by its hardness (it will easily scratch glass) and its sugary, crystalline texture that breaks across grain boundaries rather than around them. Commonly found in eroded metamorphic belts like the Appalachians or the Alps.