igneous
Rounded gray pebble, likely basalt or fine-grained volcanic rock
Probable basalt; exact identification is uncertain from the photograph
AI-generated identification · may be incorrectDark gray, dull to faintly earthy luster, apparently fine-grained and massive, with no visible cleavage; estimated hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The rounded shape indicates natural water or abrasion wear.
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Physical properties
Dark gray, dull to faintly earthy luster, apparently fine-grained and massive, with no visible cleavage; estimated hardness about 5–6 Mohs and specific gravity roughly 2.8–3.1. The rounded shape indicates natural water or abrasion wear.
Formation & geological history
Likely formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its exact age cannot be determined visually. Subsequent transport in a stream, beach, or gravel deposit rounded the specimen.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, railroad ballast, and occasionally building stone; an ordinary pebble has little commercial or jewelry value.
Geological facts
Basalt is typically composed chiefly of plagioclase feldspar and pyroxene, commonly with olivine or magnetite. A hand lens may reveal tiny interlocking crystals or scattered vesicles.
Field identification & locations
Test for a dark, dense, fine-grained rock that does not fizz with dilute hydrochloric acid and may attract a magnet weakly if magnetite-rich. Common worldwide in volcanic regions and stream gravels; distinguishing it from andesite or weathered gabbro requires a fresh surface or laboratory testing.
