igneous
Rounded basalt cobble
Basalt (fine-grained mafic volcanic rock; chiefly plagioclase and pyroxene, commonly with minor olivine and magnetite)
AI-generated identification · may be incorrectDark gray-black, fine-grained, dull to weakly vitreous and strongly weathered/rounded; hardness about 5–6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0). The smooth shape indicates water or abrasion, while tiny pale grains and rusty patches may be plagioclase and iron oxides.
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Physical properties
Dark gray-black, fine-grained, dull to weakly vitreous and strongly weathered/rounded; hardness about 5–6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0). The smooth shape indicates water or abrasion, while tiny pale grains and rusty patches may be plagioclase and iron oxides.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older volcanic terrains; this specimen was later rounded by stream, beach, or glacial transport. Exact age cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road metal, riprap, railway ballast, and building stone; attractive polished pieces may be sold as decorative specimens, but this ordinary cobble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and ocean-floor pillow lavas.
Field identification & locations
Identify it by its uniformly dark, fine texture, high density, and lack of obvious layering or large crystals; distinguish it from black shale by its massive, non-fissile character and from slag by the absence of abundant bubbles. Common worldwide, especially volcanic islands and river or beach gravels.
