igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, commonly dominated by plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, dull to weakly silky luster, very fine-grained and massive; Mohs hardness about 5–6, with no visible cleavage and typically high density (about 2.8–3.1 g/cm³). Its smooth oval shape reflects water abrasion rather than crystal form.
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Physical properties
Dark gray to black, dull to weakly silky luster, very fine-grained and massive; Mohs hardness about 5–6, with no visible cleavage and typically high density (about 2.8–3.1 g/cm³). Its smooth oval shape reflects water abrasion rather than crystal form.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. The specimen is likely geologically old relative to its recent rounding, which occurred in a stream, beach, or gravel deposit.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, building stone, and occasionally for landscaping or polished decorative objects; ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Fine grains indicate rapid cooling, while magnetic minerals may make it weakly responsive to a strong magnet.
Field identification & locations
Identify it by its dark color, heft, fine texture, and lack of obvious large crystals; a streak test is usually gray, and dilute acid produces no fizz. Common in volcanic regions and glacial or river gravels.
