sedimentary

River Cobble (Quartzite / Sandstone Gravel)

SiO2 (Quartz-rich sedimentary/metamorphic rock)

AI-generated identification · may be incorrect

Hardness: 6-7 Mohs. Color: Tan, brown, reddish-brown, grey. Luster: Dull to vitreous when wet. Texture: Smooth, well-rounded, micro-pitted surface from water erosion. Specific gravity: 2.6-2.7.

Hardness: 6-7 Mohs
Color: Tan, brown, reddish-brown, grey
Luster: Dull to vitreous when wet

Identified July 24, 2026More sedimentary →

Explore River Cobble (Quartzite / Sandstone Gravel) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 6-7 Mohs. Color: Tan, brown, reddish-brown, grey. Luster: Dull to vitreous when wet. Texture: Smooth, well-rounded, micro-pitted surface from water erosion. Specific gravity: 2.6-2.7.

Formation & geological history

Formed through the mechanical weathering, transportation, and continuous abrasion of quartz-rich rocks in riverbeds, streams, or glacial meltwater channels over thousands to millions of years.

Uses & applications

Used extensively in landscaping, river rock garden beds, aquarium decor, construction aggregate, concrete mixing, and outdoor paving.

Geological facts

Smooth river stones acquire their rounded shape from collisions with other rocks and sand in moving water, a geological process called attrition.

Field identification & locations

Identified by its egg-like or flattened oval shape, lack of sharp edges, and dense composition. Extremely common along river banks, lake shores, and glacial deposits globally.