igneous
Rounded basalt cobble
Basalt, a fine-grained mafic volcanic rock; commonly plagioclase + pyroxene ± olivine
AI-generated identification · may be incorrectDark gray-brown, dense, fine-grained, and weakly lustrous when wet, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The smooth form reflects stream or beach abrasion rather than polishing.
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Physical properties
Dark gray-brown, dense, fine-grained, and weakly lustrous when wet, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The smooth form reflects stream or beach abrasion rather than polishing.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, paving stone, and occasionally dimension stone; ordinary cobbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Weathering can produce brown iron-oxide coatings, while tiny vesicles may be obscured by the wet surface.
Field identification & locations
Look for uniformly dark color, fine texture, high density, and possible tiny feldspar crystals or vesicles; a streak is usually gray and it does not fizz with dilute hydrochloric acid. Common in volcanic regions and transported widely by rivers and glacial deposits.
