igneous
Rounded basalt river pebble
Basalt; fine-grained mafic volcanic rock, primarily plagioclase and pyroxene with minor olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to brown-black, fine-grained, opaque, and dull to slightly glassy when wet or polished; hardness about 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.0. The specimen is naturally water-rounded and may show small vesicles or weathered mineral grains.
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Physical properties
Typically dark gray to brown-black, fine-grained, opaque, and dull to slightly glassy when wet or polished; hardness about 5–6 Mohs, no visible cleavage, and specific gravity about 2.8–3.0. The specimen is naturally water-rounded and may show small vesicles or weathered mineral grains.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined visually; river rounding indicates later erosion and transport.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railroad ballast, and building stone; smooth pebbles are also used for landscaping and crafts. It has little gem value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its heavy feel, dark fine-grained texture, lack of large crystals, and weak attraction to a magnet if magnetite is present; a streak and hardness test can help distinguish it from softer sedimentary rocks. Common in volcanic regions and streambeds worldwide.
