igneous
Weathered basalt cobble
Basalt; chiefly plagioclase feldspar, clinopyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectTypically dark gray to brown-black, fine-grained, dense, and dull to subtly vitreous; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.0, with no visible cleavage in the whole rock. The rounded shape and rusty-brown surface indicate weathering and abrasion.
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Physical properties
Typically dark gray to brown-black, fine-grained, dense, and dull to subtly vitreous; hardness about 5–6 Mohs, specific gravity roughly 2.8–3.0, with no visible cleavage in the whole rock. The rounded shape and rusty-brown surface indicate weathering and abrasion.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The specimen is likely geologically ancient or young depending on its source; its current rounded form reflects stream, beach, or glacial transport.
Uses & applications
Crushed basalt is widely used as road aggregate, concrete aggregate, riprap, and railway ballast; intact cobbles are mainly decorative or educational specimens. It has little jewelry value because it lacks gem-quality crystals.
Geological facts
Its dark color results from abundant iron- and magnesium-rich minerals, while surface oxidation produces brown-red staining. Vesicles may occur, but none are obvious in this specimen.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, rusty weathering, and inability to scratch glass reliably unless fresh and hard; distinguish it from andesite by its generally darker, denser appearance. Common worldwide, especially volcanic regions and river deposits.
