igneous
Weathered brown igneous pebble, possibly basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark brown-black, mottled, dull to slightly vitreous luster, and rounded from abrasion; likely hardness about 5–6 Mohs, with no obvious cleavage. The visible texture appears dense and fine-grained, possibly containing iron-oxide staining or small vesicles.
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Physical properties
Dark brown-black, mottled, dull to slightly vitreous luster, and rounded from abrasion; likely hardness about 5–6 Mohs, with no obvious cleavage. The visible texture appears dense and fine-grained, possibly containing iron-oxide staining or small vesicles.
Formation & geological history
Likely formed when volcanic lava cooled rapidly at or near Earth’s surface, then weathered and transported in sediment. Its geological age cannot be determined from the photograph and may range from recent to hundreds of millions of years.
Uses & applications
Common basalt and andesite are used as aggregate, road stone, riprap, and building stone; an ordinary pebble has negligible commercial value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and commonly forms oceanic crust and large lava flows. Brown coloration often results from oxidation of iron-bearing minerals during weathering.
Field identification & locations
Look for a heavy, dense pebble with a fine-grained dark interior; a streak, hardness test, magnet test, and fresh broken surface would help distinguish basalt from andesite, slag, or iron-rich sedimentary rock. Common in volcanic regions and stream or beach gravels.
