Quartzite

Physical Properties

Hardness: 7 on Mohs scale; Color: Reddish-brown (due to iron oxide impurities), purple, or grey; Luster: Vitreous to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together. This typically occurs during tectonic mountain-building events.

Uses & Applications

Used in construction as crushed stone for road grit, railroad ballast, and flooring. Due to its hardness, it is also used as an abrasive and in the glass-making industry.

Geological Facts

Quartzite is so tough that when it breaks, it fractures through the quartz grains themselves rather than around the boundaries between them, unlike its parent rock, sandstone.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, sandpaper-like texture that does not rub off. Commonly found in folded mountain belts and glacial till.

Identified on: 4/28/2026

Mode: Standard