igneous
Rounded dark beach pebble, likely basalt
Fine-grained basalt, possibly andesite or dark chert
AI-generated identification · may be incorrectDark gray-brown, very fine-grained, dull to weakly satin luster, with smooth water-worn surfaces and minor pits. Estimated hardness 5.5–6.5 Mohs; cleavage absent; massive texture; specific gravity roughly 2.8–3.0 if basalt.
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Physical properties
Dark gray-brown, very fine-grained, dull to weakly satin luster, with smooth water-worn surfaces and minor pits. Estimated hardness 5.5–6.5 Mohs; cleavage absent; massive texture; specific gravity roughly 2.8–3.0 if basalt.
Formation & geological history
Most likely an exposed volcanic rock rounded by wave abrasion; basalt crystallized rapidly from mafic lava, commonly during the Cenozoic or older volcanic episodes. The exact age and composition cannot be determined from the photograph alone.
Uses & applications
Common basalt is used as aggregate, road stone, riprap, and occasionally building stone; an attractive polished pebble has modest decorative or collection value.
Geological facts
Basalt commonly contains microscopic plagioclase and pyroxene, sometimes olivine or magnetite, giving it a dark color and high density. Its smooth shape indicates prolonged transport in a beach or stream environment.
Field identification & locations
Look for a uniformly dark, dense, nonlayered stone that scratches glass weakly and typically does not react with dilute acid; a magnet may detect minor magnetite. If it has a waxier luster and conchoidal fracture, dark chert is an alternative.
