igneous
Dark volcanic rock, probably basalt
Basalt; fine-grained mafic rock composed chiefly of plagioclase and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectGray-black, fine-grained, dull to weakly vitreous surface with scattered pale mineral grains and rusty iron-oxide staining; typically hardness 6–7 Mohs, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion or transport.
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Physical properties
Gray-black, fine-grained, dull to weakly vitreous surface with scattered pale mineral grains and rusty iron-oxide staining; typically hardness 6–7 Mohs, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion or transport.
Formation & geological history
Basalt forms when low-silica mantle-derived lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age cannot be determined from the photograph and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road stone, riprap, railroad ballast, and occasionally dimension stone; generally little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Pale specks may be plagioclase phenocrysts, while rusty patches commonly represent weathered iron-bearing minerals.
Field identification & locations
Identify it by its dark, dense, fine-grained texture, lack of obvious layering, and weak attraction to a magnet if magnetite is present; a fresh broken surface and acid test help distinguish it from weathered sedimentary rocks. Common in volcanic regions and stream gravels worldwide.
