Quartzite

Hardness: 7 (Mohs scale), Color: White to grey/tan, Luster: Vitreous to dull, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. Specifically, the sandstone is subjected to intense heat and pressure, causing the original quartz grains and silica cement to recrystallize into an interlocking mosaic.

Used as crushed stone for road construction and railway ballast; also used as a dimension stone for flooring, wall coverings, and countertops. In some cases, it is a source of silica for glassmaking.

Quartzite is so hard and chemically resistant that it often forms the ridges and peaks of mountain ranges because it weathers much more slowly than surrounding rocks.

Identify in the field by its extreme hardness (it will easily scratch glass) and its tendency to break across the quartz grains rather than around them, unlike sandstone. Common in Appalachian mountains and shield areas worldwide.