igneous/sedimentary
River Pebble (Basalt/Greywacke)
Weathered Basalt or Fine-Grained Greywacke
Hardness: 6-7 (Mohs scale); Color: Dull grey, brownish-grey; Luster: Matte, earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal; Specific Gravity: 2.7-3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Dull grey, brownish-grey
- Luster
- Matte, earthy
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dull grey, brownish-grey; Luster: Matte, earthy; Crystal structure: Aphanitic (fine-grained); Cleavage: None/Conchoidal; Specific Gravity: 2.7-3.0
Formation & geological history
These are rounded stones formed through the process of fluvial erosion (water transport in rivers or streams) which smoothes sharp edges. They originate from volcanic basalt flows or compressed sedimentary layers formed over millions of years (Cenozoic to Paleozoic era).
Uses & applications
Commonly used in landscaping, decorative stone for gardens, construction aggregate, and occasionally used as 'worry stones' or for aquarium décor.
Geological facts
The rounded shape is a result of 'attrition,' where rocks collide with each other in a riverbed, wearing down the corners. Smooth pebbles like these are often indicators of ancient riverbeds or high-energy coastal environments.
Field identification & locations
Identify by the smooth, water-worn exterior and lack of visible large crystals. Commonly found in riverbeds, deltas, and beaches. Collectors should look for unique banding or perfectly spherical shapes.
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