Quartzite (River Cobble)

Physical Properties

Hardness: 7 on Mohs scale; Color: typically white to gray, often with iron staining (reddish cracks); Luster: vitreous or dull when weathered; Crystal structure: Trigonal/Hexagonal (microcrystalline); Cleavage: none, breaks with a conchoidal fracture which crosses grain boundaries.

Formation & Geological History

Formed from the metamorphism of quartz sandstone under high heat and pressure, often during mountain-building events. This specific specimen shows signs of mechanical weathering and rounding by water transport in a river or glacial environment.

Uses & Applications

Used as a high-quality building material, crushed stone for road construction, railroad ballast, and occasionally for industrial-grade glass manufacturing if high purity quartz is present.

Geological Facts

Quartzite is so tough that it often outlasts the mountain ranges it was born in, ending up as rounded river cobbles or 'erratics' transported by glaciers across vast distances.

Field Identification & Locations

Identify in the field by its hardness (it will easily scratch glass and steel) and its crystalline, sugary appearance on a fresh break. Unlike sandstone, quartzite fractures through the quartz grains rather than around them.

Identified on: 4/26/2026

Mode: Standard