sedimentary

River Cobble (Sandstone / Greywacke)

Sedimentary Rock (predominantly Quartz, Feldspar, Lithic fragments)

AI-generated identification · may be incorrect

Hardness: 6-7 Mohs scale depending on quartz content. Color: Dull gray, tan, brown, or greenish-gray with subtle color variations. Luster: Earthy to dull. Texture: Medium to fine-grained sandstone or greywacke with rounded to sub-rounded edges from stream abrasion.

Hardness: 6-7 Mohs scale depending on quartz content
Luster: Earthy to dull

Identified July 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the deposition, compaction, and cementation of mineral particles and rock fragments in river, delta, or marine environments. Later eroded and rounded by transport in streams, rivers, or glacial action.

Uses & applications

Commonly used in landscaping, river rock garden beds, erosion control, concrete aggregate, and decorative stone masonry.

Geological facts

River cobbles are smoothed over hundreds to thousands of years of natural water movement, which wears down sharp fractures and leaves a rounded, tactile surface.

Field identification & locations

Field identification: Look for rounded edges, uniform grainy texture, and matte surface finish typical of weathered sedimentary rock. Found abundantly along riverbeds, stream channels, and alluvial deposits globally.