igneous
Rounded basaltic pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to slightly earthy luster, with no visible crystals or cleavage; typically Mohs hardness 5–6 and specific gravity about 2.8–3.0. Its smooth, elongated shape reflects abrasion by water or sediment transport.
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Physical properties
Dark gray-brown, fine-grained, dull to slightly earthy luster, with no visible crystals or cleavage; typically Mohs hardness 5–6 and specific gravity about 2.8–3.0. Its smooth, elongated shape reflects abrasion by water or sediment transport.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic events; the specimen was later rounded in a stream, beach, or glacial environment. Exact age and locality cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally landscaping material; attractive polished pieces may be used decoratively but have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms from mantle-derived magma. Fine texture indicates rapid cooling, while the brown coloration likely reflects surface weathering and iron oxidation.
Field identification & locations
Identify it by its uniform fine grain, heavy feel, dark mafic color, and lack of obvious layering or large crystals; a streak, magnetism, density, and thin-section test would improve confidence. Common worldwide, especially near volcanic terrains, river gravels, and beaches.
