Mixed: Igneous and Sedimentary

River Cobbles (Basalt, Quartzite, and Sandstone mix)

Fluvial detritus (primarily SiO2, Plagioclase, and Pyroxene mix)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs); Colors: gray, tan, dark blue-gray, off-white; Luster: dull to earthy; Structure: rounded/sub-rounded by water erosion; Cleavage: none (fractures irregularly); Specific Gravity: 2.5-3.0.

Hardness: 6-7 (Mohs)
Luster: dull to earthy

Identified March 19, 2026More mixed: igneous and sedimentary →

Explore River Cobbles (Basalt, Quartzite, and Sandstone mix) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through various processes (volcanic cooling for basalt, sedimentation for sandstone) and subsequently weathered and transported by high-energy river systems or glacial movement over thousands to millions of years.

Uses & applications

Landscaping, decorative garden 'cairns', construction aggregate, road base, and aquarium decor.

Geological facts

The smooth, rounded shape is a result of fluvial abrasion, where rocks tumble against each other in moving water. These specific stacked rocks are arranged in a 'cairn' style, historically used as trail markers or spiritual symbols.

Field identification & locations

Identify by a smooth, water-worn exterior and lack of sharp edges. Commonly found in riverbeds, glacial moraines, and coastal beaches. Collectors should look for unique banding or internal crystal textures revealed by polishing.