igneous
Basaltic volcanic pebble
Fine-grained mafic igneous rock, chiefly plagioclase and pyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, dull to weakly granular, typically fine-grained and massive; Mohs hardness about 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion, and the surface may show minor weathering or vesicles.
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Physical properties
Dark gray to black, dull to weakly granular, typically fine-grained and massive; Mohs hardness about 5–6, no visible cleavage, and specific gravity about 2.8–3.0. The rounded shape indicates water abrasion, and the surface may show minor weathering or vesicles.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly during volcanic eruptions; individual pieces may range from geologically young to hundreds of millions of years old. This pebble was likely transported in a stream or beach environment.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally dimension stone; ordinary pebbles have negligible jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the oceanic crust. Positive identification would require a fresh broken surface or testing, because dark andesite, gabbro, and slag can look similar.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits. Look for a dense black fine-grained texture and a gray streak; a magnet may respond weakly due to magnetite, while strong vesicles or glassiness suggest related volcanic material.
