igneous
Rounded basalt cobble
Basalt, aphanitic mafic volcanic rock; commonly plagioclase + clinopyroxene ± olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray to brown, weathered surface with scattered pale crystals or vesicle-like pits; dull to faintly lusterous, very fine-grained, massive, and lacking visible cleavage. Hardness about 6–7 Mohs; typically dense, specific gravity 2.8–3.0.
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Physical properties
Dark gray to brown, weathered surface with scattered pale crystals or vesicle-like pits; dull to faintly lusterous, very fine-grained, massive, and lacking visible cleavage. Hardness about 6–7 Mohs; typically dense, specific gravity 2.8–3.0.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, likely during the Cenozoic or older depending on its source; its smooth, oval shape indicates prolonged stream or beach abrasion.
Uses & applications
Used as aggregate, road stone, riprap, landscaping rock, and occasionally as a source material for basalt fiber; generally unsuitable for jewelry except as a decorative cabochon or specimen.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of oceanic crust and many volcanic plateaus.
Field identification & locations
Identify by its dark, fine-grained texture, high density, and resistance to scratching; a glassy streak or strong reaction to acid would suggest another material. Common in volcanic terrains and downstream gravel deposits.
