igneous
Rounded basalt cobble
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to weakly vitreous, with weathered reddish-brown patches and small vesicles or pits; hardness about 6 Mohs, massive texture, no obvious cleavage, and specific gravity roughly 2.8–3.0
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Physical properties
Dark gray-brown, fine-grained, dull to weakly vitreous, with weathered reddish-brown patches and small vesicles or pits; hardness about 6 Mohs, massive texture, no obvious cleavage, and specific gravity roughly 2.8–3.0
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, likely in the Cenozoic or older depending on its source; the rounded shape indicates prolonged river, beach, or glacial abrasion
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and occasionally landscaping material; generally modest collector value unless linked to a notable locality
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, volcanic islands, and seafloor pillow lavas
Field identification & locations
Identify it by its heavy feel, dark fine-grained matrix, iron staining, and lack of visible large crystals; common worldwide, especially volcanic regions and stream or beach gravels. Confirm with a streak, magnetism test, and thin-section or chemical analysis if precise identification is needed
