igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray to brown, massive, fine-grained, and dull to weakly submetallic where weathered; typically Mohs 6, with no visible cleavage and specific gravity about 2.8–3.0. Its smooth shape indicates prolonged water or sediment abrasion.
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Physical properties
Dark gray to brown, massive, fine-grained, and dull to weakly submetallic where weathered; typically Mohs 6, with no visible cleavage and specific gravity about 2.8–3.0. Its smooth shape indicates prolonged water or sediment abrasion.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. The specimen’s precise age and source cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and building stone; attractive polished pieces are sold as decorative or lapidary material.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Iron-bearing minerals commonly weather to reddish-brown staining like that visible here.
Field identification & locations
Identify it by its uniformly fine texture, dark color, high density, and lack of large crystals; a streak, magnetism, and thin-section test would improve certainty. Common worldwide, especially near volcanic terrain and streambeds.
