Quartzite (River Cobble)

Physical Properties

Hardness: 7 (Mohs scale); Color: White, cream, or light tan with some iron staining; Luster: Dull to vitreous; Crystal structure: Trigonal (granular texture); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.8.

Formation & Geological History

Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure, causing the original quartz grains to recrystallize into a dense, interlocking mosaic of crystals. This specific specimen has been rounded by fluvial (river) erosion.

Uses & Applications

Used as a construction aggregate, for railway ballast, as a decorative stone in landscaping, and in the production of glass or silica-based products.

Geological Facts

Quartzite is so hard and chemically resistant that it often survives as resistant ridges in mountain ranges or as durable pebbles in river beds long after other rocks have eroded away.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel blade) and its granular, sugary appearance which distinguishes it from massive quartz. It is commonly found in riverbanks and glacial till.

Identified on: 4/19/2026

Mode: Standard