igneous
Dark weathered pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and iron oxides)
AI-generated identification · may be incorrectDark gray-black, fine-grained, dull to subtly granular luster, with no visible large crystals or cleavage; typical hardness about 6 on Mohs scale and specific gravity 2.8–3.0. The pale web-like surface may be weathering, mineral veining, or lichen rather than diagnostic texture.
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Physical properties
Dark gray-black, fine-grained, dull to subtly granular luster, with no visible large crystals or cleavage; typical hardness about 6 on Mohs scale and specific gravity 2.8–3.0. The pale web-like surface may be weathering, mineral veining, or lichen rather than diagnostic texture.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, generally in volcanic settings and oceanic crust. Its age cannot be determined from the photograph; basalt ranges from very young lava to Precambrian examples.
Uses & applications
Used as crushed aggregate, roadstone, riprap, building stone, and occasionally polished for decorative objects. An ordinary pebble has little commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, magnetic response, and microscopic crystals commonly distinguish it from similar dark stones.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits worldwide; test for a faint magnetism and inspect a fresh break for a uniform fine-grained texture. Identification is provisional without hardness, streak, and geological context.
