igneous or metamorphic

River Cobble (likely Basalt or Metagraywacke)

Non-specific water-worn lithic fragment

AI-generated identification · may be incorrect

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

Identified April 12, 2026More igneous or metamorphic →

Explore River Cobble (likely Basalt or Metagraywacke) in the encyclopedia →

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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).