Quartzite River Cobble

Hardness 7 on Mohs scale; color ranges from tan, reddish-pink (iron staining), to grey; dull to slightly vitreous luster; granular crystalline texture; no cleavage; dense with a specific gravity of ~2.65.

Formed from quartz-rich sandstone subjected to high temperatures and pressures during regional metamorphism, fusing individual quartz sand grains into an interlocking mosaic of quartz crystals. Subsequently eroded, transported, and rounded by fluvial or glacial action.

Commonly used in construction as crushed stone, railway ballast, aggregate for concrete, and decorative landscaping rock.

Quartzite is so tough and resistant to weathering that it often breaks straight through quartz grains rather than along the boundaries between them (unlike sandstone). River tumbling smooths its surfaces into ergonomic, dense cobbles.

Identifiable by its gritty appearance yet smooth, hard surface that readily scratches glass and steel without scratching from a standard pocket knife. Abundantly found in river beds, glacial deposits, and gravel bars worldwide.