sedimentary
River Cobble (Sandstone / Greywacke)
Sedimentary Rock (predominantly Quartz, Feldspar, Lithic fragments)
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
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Physical properties
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.
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.
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