igneous
Basalt cobble
Basalt; predominantly plagioclase feldspar, clinopyroxene, and Fe-Mg minerals
AI-generated identification · may be incorrectDark gray to black, fine-grained, massive, and moderately heavy, with a dull to locally glassy/waxy luster. Typical hardness is about 6, with no obvious cleavage; the rounded shape and smooth surface suggest water wear or polishing.
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Physical properties
Dark gray to black, fine-grained, massive, and moderately heavy, with a dull to locally glassy/waxy luster. Typical hardness is about 6, with no obvious cleavage; the rounded shape and smooth surface suggest water wear or polishing.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Its rounded form likely developed in a river, beach, or glacial environment.
Uses & applications
Used as crushed aggregate, roadstone, riprap, ballast, and landscaping stone; attractive polished pieces are collected as curiosities. It has little gemstone value unless it contains distinctive vesicles, crystals, or alteration minerals.
Geological facts
The specimen may contain iron-oxide staining or weathered mafic minerals producing the brown patches. A magnet may show weak attraction if magnetite is present.
Field identification & locations
Identify basalt by its uniformly dark, fine texture, high density, and lack of visible large crystals; it is common in volcanic regions, oceanic crust, Hawaii, Iceland, and flood-basalt provinces. A streak test is usually gray to black, and it should not fizz with dilute hydrochloric acid.
