igneous
Likely basaltic rock fragment
Fine-grained mafic igneous rock, probably basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray-black, dull to slightly granular, irregular and very fine-grained; typically Mohs 5–6, with no visible cleavage and a dense texture. The apparent pale speckling may be weathered crystals or vesicle fillings.
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Physical properties
Dark gray-black, dull to slightly granular, irregular and very fine-grained; typically Mohs 5–6, with no visible cleavage and a dense texture. The apparent pale speckling may be weathered crystals or vesicle fillings.
Formation & geological history
Basalt forms when low-silica magma cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and cinder deposits; specimens may range from modern to Precambrian in age.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; small unremarkable fragments have little jewelry value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the ocean floor. A definitive identification requires a fresh break, streak, magnetism, density, and ideally thin-section or chemical testing.
Field identification & locations
Common worldwide, especially volcanic regions, oceanic islands, and dark gravel. Distinguish it from slag or meteorite by its natural crystalline texture, lack of metallic droplets, and usually non-magnetic to moderately magnetic behavior.
