igneous and sedimentary
River Rock (Basalt/Greywacke pebbles)
Fluvial gravel primarily composed of Mafic Volcanic Rock (Basalt) or Lithic Sandstone (Greywacke)
Hardness: 6-7 (Mohs scale); Color: charcoal grey, olive green, dark brown; Luster: dull to waxy (when wet); Crystal structure: aphanitic or fine-grained; Cleavage: none; Specific gravity: 2.6 - 3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- charcoal grey, olive green, dark brown
- Luster
- dull to waxy (when wet)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: charcoal grey, olive green, dark brown; Luster: dull to waxy (when wet); Crystal structure: aphanitic or fine-grained; Cleavage: none; Specific gravity: 2.6 - 3.0
Formation & geological history
Formed through volcanic cooling (basalt) or high-pressure sediment deposition (greywacke). They acquire their rounded, smooth shape through fluvial erosion (water transport in rivers) over thousands of years.
Uses & applications
Landscaping, drainage systems, construction aggregate, aquarium substrate, and decorative pathways.
Geological facts
River rocks are 'clastic' in origin, meaning they are broken pieces of older rocks. Most dark river stones like these are sourced from glacial deposits or active riverbeds in the Pacific Northwest or Appalachia.
Field identification & locations
Identify in the field by their smooth, rounded edges and lack of sharp points. Frequently found in riverbeds, lake shores, and glacial moraines. Collectors look for unique banding or 'lucky stones' with white quartz veins.
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