igneous
Rounded basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray to brown, weathered, and faintly granular with a dull to subvitreous luster; hardness about 6 Mohs, no obvious cleavage, and typical specific gravity of 2.8–3.0. Its rounded shape indicates prolonged abrasion, likely in a stream or beach environment.
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Physical properties
Dark gray to brown, weathered, and faintly granular with a dull to subvitreous luster; hardness about 6 Mohs, no obvious cleavage, and typical specific gravity of 2.8–3.0. Its rounded shape indicates prolonged abrasion, likely in a stream or beach environment.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the specimen’s exact age cannot be determined visually. Surface oxidation and weathering produce the brown coloration.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and occasionally as landscaping stone or a carving medium; this small cobble has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and ocean-floor pillow lavas. Vesicles may occur, but none are clearly visible here.
Field identification & locations
Identify it by its dark color, heavy feel, fine interlocking texture, and lack of visible quartz or large crystals; common sources include volcanic regions, river gravels, and beaches. A streak, magnetism test, and fresh broken surface would improve identification.
