Quartzite

Physical Properties

Hardness: 7 (Mohs scale); Color: White, gray, or tan; Luster: Vitreous to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.7

Formation & Geological History

Formed through the metamorphism of quartz-rich sandstone. High heat and pressure cause the quartz grains to recrystallize and fuse together, forming a dense, durable rock. This process can occur over various geological ages, often in mountain-building (orogenic) zones.

Uses & Applications

Used widely in construction as crushed stone for road ballast, roofing granules, and as a decorative stone in architecture. High-purity quartzite is a source of silica for glassmaking and silicon production. Some varieties are used as semi-precious stones for jewelry.

Geological Facts

Quartzite is so strong that when it breaks, it fractures through the quartz grains rather than along the boundaries between them, unlike its parent stone, sandstone. It is often resistant to chemical weathering and creates prominent ridges in mountain ranges.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its sugary, crystalline texture. Commonly found in folded mountain belts like the Appalachians or the Alps. For collectors, look for specimens with interesting mineral inclusions or vibrant colors caused by iron oxides.

Identified on: 4/13/2026

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