igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock
AI-generated identification · may be incorrectTypically 5–6 on the Mohs scale, dark gray to greenish-black, dull to subtly waxy, and very fine-grained; chiefly composed of plagioclase feldspar, pyroxene, and often olivine. The specimen appears water-worn, with no obvious cleavage or visible crystals.
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Physical properties
Typically 5–6 on the Mohs scale, dark gray to greenish-black, dull to subtly waxy, and very fine-grained; chiefly composed of plagioclase feldspar, pyroxene, and often olivine. The specimen appears water-worn, with no obvious cleavage or visible crystals.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph and may range from recent to billions of years old.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and dimension stone; attractive polished pieces are occasionally sold as décor or collectibles.
Geological facts
Its rounded shape and smooth surface likely result from stream or beach transport. A magnet may show weak attraction if the basalt contains abundant magnetite.
Field identification & locations
Identify it by its uniform fine texture, relatively high density, dark color, and lack of visible quartz or large crystals; a streak is usually gray. Common worldwide, especially near volcanic regions and river gravels.
