igneous
Dark basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically very fine-grained, dark gray to black, with a dull to faintly satin luster; hardness about 5–6 Mohs, dense, tough, and lacking visible cleavage. The smoothly rounded shape and uniform surface indicate water or wave abrasion.
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Physical properties
Typically very fine-grained, dark gray to black, with a dull to faintly satin luster; hardness about 5–6 Mohs, dense, tough, and lacking visible cleavage. The smoothly rounded shape and uniform surface indicate water or wave abrasion.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and continental lava flows. Individual flows range from ancient Precambrian to modern eruptions; the specimen’s age cannot be determined visually.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and occasionally building stone or landscaping material. This ordinary cobble has little jewelry or collector value.
Geological facts
Its dark color reflects abundant iron- and magnesium-rich minerals; weathering commonly produces reddish-brown iron staining. It may be weakly magnetic if it contains magnetite.
Field identification & locations
Identify it by its uniform dark fine texture, high density, and absence of visible large crystals or layering; a streak, hand lens, and magnet can help distinguish it from slate, slag, or dark chert. Common worldwide near volcanic terrains and river or beach gravels.
