igneous
Pebble of weathered basalt or andesite
Fine- to medium-grained mafic volcanic rock; commonly plagioclase ± pyroxene, olivine, and iron oxides
AI-generated identification · may be incorrectDark brown to black, strongly weathered, with dull to locally submetallic luster and scattered pale crystals/vesicles. Typical hardness is about 5–6 Mohs; massive, nonfoliated, with no obvious cleavage; specific gravity roughly 2.7–3.1.
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Physical properties
Dark brown to black, strongly weathered, with dull to locally submetallic luster and scattered pale crystals/vesicles. Typical hardness is about 5–6 Mohs; massive, nonfoliated, with no obvious cleavage; specific gravity roughly 2.7–3.1.
Formation & geological history
Likely an erosion-rounded volcanic rock formed when lava cooled rapidly at or near Earth’s surface, then altered by weathering and transport. Its exact geological age cannot be determined from the photograph.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, and occasionally building stone; attractive pieces may be sold as landscape or display specimens. This example appears to have low commercial value.
Geological facts
The elongated, rounded shape indicates abrasion in a stream, beach, or glacial environment. Brown coloration likely reflects oxidation of iron-bearing minerals rather than the original fresh-rock color.
Field identification & locations
Look for a dense dark groundmass, small plagioclase crystals, magnetic response, and lack of layering; a streak, hardness, and thin-section test would distinguish basalt from andesite or altered sedimentary rock. Basalt pebbles occur worldwide in volcanic terrains and river gravels.
