igneous
Rounded dark pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectDark gray to black, dull to subtly waxy luster, fine-grained and massive; typically hardness 5–6 Mohs, with no visible cleavage and specific gravity about 2.8–3.0. The smooth shape indicates water or sediment abrasion, and the pale patches may be weathering or minor mineral inclusions.
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Physical properties
Dark gray to black, dull to subtly waxy luster, fine-grained and massive; typically hardness 5–6 Mohs, with no visible cleavage and specific gravity about 2.8–3.0. The smooth shape indicates water or sediment abrasion, and the pale patches may be weathering or minor mineral inclusions.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. 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, building stone, and occasionally as a polished decorative material; ordinary pebbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It commonly attracts weakly to moderately to a magnet if it contains magnetite.
Field identification & locations
Identify it by its uniformly fine texture, dark color, heavy feel, and lack of obvious quartz or large crystals; a streak, magnet, and acid test can help distinguish it from dark slate or chert. Common sources include volcanic regions, riverbeds, beaches, and glacial deposits.
