igneous
Dark mafic rock, likely basalt
Basalt; mainly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectTypically black to dark gray, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.0. The specimen appears weathered and possibly weakly vesicular, but a photograph cannot confirm exact mineralogy.
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Physical properties
Typically black to dark gray, fine-grained, massive, and dull to subtly vitreous; Mohs hardness about 5–6, with no visible cleavage and specific gravity roughly 2.8–3.0. The specimen appears weathered and possibly weakly vesicular, but a photograph cannot confirm exact mineralogy.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age may range from modern eruptions to Precambrian examples.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, building stone, and occasionally polished architectural stone; ordinary pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly weakly magnetic when magnetite-rich.
Field identification & locations
Identify it by its dense dark color, fine texture, and lack of large crystals; test for a weak magnetic response and compare hardness with steel. Common worldwide, especially Hawaii, Iceland, India’s Deccan Traps, and oceanic islands.
