igneous
Rounded dark pebble, likely basalt
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark gray to black, smooth weathered surface, dull to faintly vitreous luster, very fine-grained structure, and no visible cleavage. The shape indicates water abrasion or tumbling.
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Physical properties
Hardness about 6–7 Mohs; dark gray to black, smooth weathered surface, dull to faintly vitreous luster, very fine-grained structure, and no visible cleavage. The shape indicates water abrasion or tumbling.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; the specimen was later rounded by erosion. Exact identification and age require testing or microscopy.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and dimension stone; attractive polished pieces are sold as inexpensive decorative specimens.
Geological facts
Basalt is the most abundant volcanic rock in oceanic crust and commonly forms pillow lavas underwater. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Look for a dense, dark, fine-grained pebble with no layering or obvious large crystals; a streak, magnetism, density, or thin-section test can distinguish it from dark chert or slag. Common near volcanic regions, river gravels, beaches, and glacial deposits.
