igneous
Rounded gray river pebble, likely fine-grained basalt or andesite
Basaltic-andesitic volcanic rock; commonly plagioclase feldspar, pyroxene, and minor iron oxides
AI-generated identification · may be incorrectDark gray, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; typically Mohs 6–7 and specific gravity about 2.7–3.1. Its smooth shape reflects prolonged water or beach abrasion.
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Physical properties
Dark gray, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; typically Mohs 6–7 and specific gravity about 2.7–3.1. Its smooth shape reflects prolonged water or beach abrasion.
Formation & geological history
An extrusive volcanic rock formed when mafic to intermediate lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older volcanic terrains; the pictured pebble was later transported and rounded.
Uses & applications
Commonly used as aggregate, road stone, riprap, landscaping material, and occasionally for carving or polished decorative pieces; usually little jewelry value.
Geological facts
The exact identification cannot be confirmed from a photograph alone; similar gray pebbles may be fine-grained metamorphic rock or dense sedimentary rock.
Field identification & locations
Look for a uniformly fine texture, heavy feel, lack of layering, and possible tiny white feldspar specks; a streak, magnetism test, hardness test, and fresh broken surface improve identification. Common in volcanic regions and stream or coastal gravels.
