sedimentary
River Cobble (Mixed Lithology)
Clastic Sedimentary Rock (Silicified Siltstone or Wacke)
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
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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.
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