igneous
Rounded river pebble, likely basalt or andesite
Fine-grained mafic volcanic rock; commonly plagioclase ± pyroxene, olivine, and iron oxides
AI-generated identification · may be incorrectDark gray-brown, massive, fine-grained, and weathered with a dull to faintly satin luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.1.
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Physical properties
Dark gray-brown, massive, fine-grained, and weathered with a dull to faintly satin luster; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.1.
Formation & geological history
Likely an extrusive volcanic rock formed when lava cooled rapidly at or near Earth’s surface, then was rounded by stream or beach abrasion; its precise age and provenance cannot be determined from the photograph.
Uses & applications
Used as aggregate, road stone, landscaping rock, and occasionally as a lapidary or decorative pebble; this ordinary specimen has little commercial value.
Geological facts
The smooth shape is transport-related rather than a crystal habit. A concealed fresh surface may reveal a uniformly dark gray interior, while weathering commonly produces brown or rusty coatings.
Field identification & locations
Common in volcanic terrains and alluvial deposits worldwide. Test for a fine-grained texture, relatively high density, lack of acid reaction, and possible weak attraction to a strong magnet; a scratch test can distinguish it from softer sedimentary rocks.
