igneous
Small tumbled dark pebble, likely basalt or andesite
Fine-grained mafic/intermediate volcanic rock; commonly basalt (plagioclase + pyroxene ± olivine)
AI-generated identification · may be incorrectGray-black, rounded and slightly dull, with possible lighter mineral grains and weathered brown patches. Typical hardness is about 5–6 Mohs; massive, non-cleavable, and fine-grained, though the photograph is insufficient for a definitive identification.
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Physical properties
Gray-black, rounded and slightly dull, with possible lighter mineral grains and weathered brown patches. Typical hardness is about 5–6 Mohs; massive, non-cleavable, and fine-grained, though the photograph is insufficient for a definitive identification.
Formation & geological history
Likely formed when lava cooled rapidly at or near Earth’s surface, then became rounded by stream or beach transport. Basalt and andesite range from Precambrian to modern geological ages and occur in volcanic terrains worldwide.
Uses & applications
Used as aggregate, roadstone, ballast, riprap, and occasionally as a decorative or lapidary stone. This small specimen has little commercial value.
Geological facts
The specimen’s rounded shape indicates abrasion after formation, while its dark color suggests abundant iron- and magnesium-bearing minerals. A magnet test, streak test, and close examination of crystal grains would improve identification.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and landscaping stone. Estimated collector value: $1–$5 for an ordinary pebble, or up to about $10 if unusually attractive or locality-labeled.
