igneous
Rounded basalt cobble
Basalt, a fine-grained mafic volcanic rock; commonly composed of plagioclase, pyroxene, and minor olivine
AI-generated identification · may be incorrectDark gray-green, dense, matte to faintly dull luster, with a fine-grained texture and no visible crystals; hardness about 5–6 Mohs, typically massive with no obvious cleavage, and specific gravity about 2.8–3.0.
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Physical properties
Dark gray-green, dense, matte to faintly dull luster, with a fine-grained texture and no visible crystals; hardness about 5–6 Mohs, typically massive with no obvious cleavage, and specific gravity about 2.8–3.0.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, often during the Cenozoic or older volcanic episodes; its smooth shape and rounded edges indicate later abrasion in a river, beach, or glacial environment.
Uses & applications
Common basalt is used for crushed aggregate, road stone, riprap, ballast, and occasionally landscaping; this rounded specimen has mainly educational or decorative value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. The apparent dark spots may be weathering stains or small vesicles rather than distinct minerals.
Field identification & locations
Identify it by its dark color, high density, very fine grain, and lack of layering; a streak test is usually gray, and it generally does not react with dilute hydrochloric acid. Common worldwide, especially near volcanic terrains and river or beach gravels.
