igneous
Rounded dark cobble, likely basalt
Basalt (fine-grained mafic volcanic rock; chiefly plagioclase and pyroxene, with possible olivine/magnetite)
AI-generated identification · may be incorrectDark green-black, weathered and water-rounded, with a dull to weakly glassy luster; typically Mohs 5–6, dense, fine-grained, and without visible cleavage or large crystals.
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Physical properties
Dark green-black, weathered and water-rounded, with a dull to weakly glassy luster; typically Mohs 5–6, dense, fine-grained, and without visible cleavage or large crystals.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces; the specimen’s age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and dimension stone; attractive polished pieces may be used decoratively, though this cobble has little gem value.
Geological facts
The greenish coloration may reflect chlorite, epidote, or weathering of olivine, while rusty patches commonly indicate iron oxidation.
Field identification & locations
Identify basalt by its dark color, high density, fine texture, and inability to scratch easily with a steel knife; common in volcanic and glacial/stream gravels worldwide. A definitive identification requires a fresh surface or petrographic testing.
