igneous
Likely basalt
Mafic volcanic rock, chiefly plagioclase and pyroxene, commonly with minor olivine and magnetite
AI-generated identification · may be incorrectDark gray to black, fine-grained, locally vesicular, with a dull to weakly vitreous luster and scattered reflective mineral grains. Hardness is about 6 on Mohs; massive basalt has no cleavage, and its specific gravity is typically 2.7–3.0.
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Physical properties
Dark gray to black, fine-grained, locally vesicular, with a dull to weakly vitreous luster and scattered reflective mineral grains. Hardness is about 6 on Mohs; massive basalt has no cleavage, and its specific gravity is typically 2.7–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, producing tiny crystals; it may be billions of years old or geologically recent. The image could also represent a dark, weathered volcanic rock, but texture alone cannot confirm the identification.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, building stone, and occasionally polished decorative material. Ordinary specimens have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of oceanic crust and many large lava plateaus. Vesicles may later be filled by zeolite, calcite, or quartz.
Field identification & locations
Look for a dense black fine-grained matrix, tiny pale plagioclase crystals, magnetic response, and occasional gas holes; it commonly occurs in lava flows, volcanic terrains, and oceanic crust. A hand lens and streak/magnet tests help distinguish it from slag.
