igneous
Rounded porphyritic basalt (vesicular volcanic rock)
Basalt, likely olivine- and pyroxene-bearing; approximate composition: plagioclase + clinopyroxene ± olivine and iron oxides
AI-generated identification · may be incorrectDark gray to brown, fine-grained groundmass with small pale or rusty crystals and possible vesicles; dull to subtly glassy luster, no obvious cleavage, and Mohs hardness about 5–6. The rounded shape indicates stream or beach abrasion.
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Physical properties
Dark gray to brown, fine-grained groundmass with small pale or rusty crystals and possible vesicles; dull to subtly glassy luster, no obvious cleavage, and Mohs hardness about 5–6. The rounded shape indicates stream or beach abrasion.
Formation & geological history
Extrusive mafic lava cooled rapidly at or near Earth’s surface, probably Cenozoic or older; brown coloration may reflect weathering and iron oxidation. Exact age and locality cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and occasionally building stone; this rounded specimen has mainly educational or decorative value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust, including seafloor pillow lavas and many volcanic islands.
Field identification & locations
Identify by its dark fine-grained texture, high density, interlocking crystals, and lack of strong layering; a hand lens may reveal plagioclase laths, pyroxene, or olivine. Common worldwide in volcanic terrains and river or beach gravels.
