igneous
Rounded gray pebble, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact identification requires testing
AI-generated identification · may be incorrectDark gray-green, dull to weakly waxy luster, fine-grained and massive, with no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.0. The rounded shape and smooth surface indicate water or abrasion wear.
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Physical properties
Dark gray-green, dull to weakly waxy luster, fine-grained and massive, with no visible cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.7–3.0. The rounded shape and smooth surface indicate water or abrasion wear.
Formation & geological history
Likely formed when lava cooled rapidly at or near Earth’s surface, then was transported and rounded in a stream, beach, or glacial deposit. Its exact geological age and setting cannot be determined from the photograph.
Uses & applications
Common basalt and andesite are used as aggregate, road stone, riprap, paving, and occasionally decorative stone; this small pebble has negligible commercial value.
Geological facts
Basalt is usually richer in iron and magnesium and darker than andesite, while andesite is commonly gray and slightly lighter. A fresh broken surface, streak, density, and magnetic response would help distinguish them.
Field identification & locations
Look for a uniformly fine texture, dark color, and absence of sedimentary layering or obvious crystals; basalt may be weakly magnetic. Common sources include volcanic terrains and river or beach gravels worldwide.
