igneous
Rounded gray pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase feldspar, pyroxene, and possible olivine
AI-generated identification · may be incorrectGray, dull to faintly silky surface with a fine-grained texture and rounded, water-worn shape; individual crystals are not visible. Typical hardness is about 5–6 Mohs, with no obvious cleavage and specific gravity roughly 2.7–3.1.
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Physical properties
Gray, dull to faintly silky surface with a fine-grained texture and rounded, water-worn shape; individual crystals are not visible. Typical hardness is about 5–6 Mohs, with no obvious cleavage and specific gravity roughly 2.7–3.1.
Formation & geological history
Likely formed when volcanic lava cooled rapidly at or near Earth’s surface, then was rounded by stream or beach transport. Its exact geological age cannot be determined from the photograph; basaltic rocks range from ancient to modern.
Uses & applications
Common basalt and andesite are used as crushed aggregate, road stone, riprap, and building material; attractive polished pieces may be used decoratively. This ordinary pebble has little commercial value.
Geological facts
The pale gray color may indicate andesite, weathered basalt, or a volcanic rock containing abundant feldspar. A close examination or thin section would be needed for confident identification.
Field identification & locations
Look for a dense, fine-grained texture, lack of visible layering, and possible tiny feldspar specks; a streak test, magnetism test, and fresh broken surface can help distinguish basalt from andesite. Common in volcanic regions, riverbeds, beaches, and glacial deposits.
