Quartzite pebble

Hardness 7 on the Mohs scale, vitreous to dull luster, granular sugary fracture texture, whitish-gray to pale beige color, no cleavage, specific gravity ~2.65.

Formed from quartz-rich sandstone subjected to high heat and pressure during regional or contact metamorphism, recrystallizing into an interlocking mosaic of quartz grains. Later shaped and smoothed by fluvial or glacial weathering.

Commonly crushed for construction aggregate, railway ballast, riprap, and silica source for glassmaking. Smooth river pebbles are used in landscaping.

Quartzite is so tough that when it breaks, fractures propagate through the individual quartz grains rather than along the cement between them, unlike sandstone.

Identified in the field by its extreme hardness (easily scratches glass and unhardened steel), sugary texture, and lack of reaction with hydrochloric acid. Very common in riverbeds and glacial deposits worldwide.