igneous or metamorphic
River Cobble (likely Basalt or Metagraywacke)
Non-specific water-worn lithic fragment
Hardness: 5-7 (Mohs); Color: Dark grey, greenish-grey, or black; Luster: Dull to waxy when wet; Crystal structure: Fine-grained (aphanitic); Cleavage: Indistinct; Specific gravity: Approx 2.7-3.0
- Hardness
- 5-7 (Mohs)
- Color
- Dark grey, greenish-grey, or black
- Luster
- Dull to waxy when wet
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Physical properties
Hardness: 5-7 (Mohs); Color: Dark grey, greenish-grey, or black; Luster: Dull to waxy when wet; Crystal structure: Fine-grained (aphanitic); Cleavage: Indistinct; Specific gravity: Approx 2.7-3.0
Formation & geological history
Formed through volcanic activity (if igneous) or heat/pressure (if metamorphic). This specific specimen has been significantly modified by fluvial erosion, rounding it over hundreds or thousands of years in a stream or river bed.
Uses & applications
Common river rocks are used in landscaping, drainage systems, construction aggregates, and occasionally as decorative aquarium stones.
Geological facts
Rocks like this are key indicators for prospectors; the presence of heavy, rounded stones in a stream bed often marks areas where gold or other heavy minerals may settle in 'placer' deposits.
Field identification & locations
Identify by its rounded, smooth surface and heavy weight relative to size. Found in riverbeds, glacial tills, and alluvial plains. Collectors value them for lapidary work (tumbling).
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