sedimentary

River Cobble (Mixed Lithology)

Clastic Sedimentary Rock (Silicified Siltstone or Wacke)

AI-generated identification · may be incorrect

Hardness: 6-7 (Mohs scale); Color: Muted grey, tan, and dark charcoal; Luster: Dull/Earthy; Crystal structure: Microcrystalline to aphanitic; Cleavage: None/Conchoidal fracture.

Hardness: 6-7 (Mohs scale)
Color: Muted grey, tan, and dark charcoal
Luster: Dull/Earthy

Identified July 4, 2026More sedimentary →

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Physical properties

Hardness: 6-7 (Mohs scale); Color: Muted grey, tan, and dark charcoal; Luster: Dull/Earthy; Crystal structure: Microcrystalline to aphanitic; Cleavage: None/Conchoidal fracture.

Formation & geological history

Formed from the lithification of fine-grained sediment (silt or sand) in a marine or fluvial environment, followed by significant physical weathering and transport in a river system which rounded the edges.

Uses & applications

Used in landscaping, construction aggregate, road base, and as decorative river stones.

Geological facts

The rounded shape indicates a long history of travel through water, where constant tumbling against other rocks smoothed its surface. This specific specimen shows dark mineral staining, likely manganese or iron oxides.

Field identification & locations

Identify in the field by its smooth, water-worn texture and lack of visible large crystals. Commonly found in stream beds, glacial tills, and coastal regions.