igneous
Rounded mafic river stone, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and iron-titanium oxides)
AI-generated identification · may be incorrectDark brown-gray, fine-grained, dull to subtly granular, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. Its smooth, triangular form indicates prolonged water or abrasion weathering.
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Physical properties
Dark brown-gray, fine-grained, dull to subtly granular, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. Its smooth, triangular form indicates prolonged water or abrasion weathering.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly during volcanic eruptions, seafloor spreading, or hotspot activity. The specimen’s geological age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road material, riprap, railway ballast, and occasionally dimension stone. This ordinary rounded piece has little jewelry or decorative value.
Geological facts
The surface appears weathered and possibly coated with fine iron-oxide staining, which produces the brown coloration. Fine texture and overall dark mafic appearance distinguish it from coarse granite.
Field identification & locations
Look for a uniformly fine texture, dark color, high density, and absence of obvious quartz or large feldspar crystals; a streak test is usually gray to dark gray. Common in volcanic regions and gravel bars worldwide.
