igneous
Rounded river cobble, likely basalt or weathered andesite
Fine-grained mafic volcanic rock; likely basalt (plagioclase ± pyroxene/olivine)
AI-generated identification · may be incorrectDark olive-brown, smooth and subrounded from abrasion, with dull-to-faintly waxy luster and no visible crystals; estimated Mohs hardness 5–6.5, massive texture, and negligible cleavage. The brown-yellow surface coloration likely reflects weathering or iron-oxide staining.
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Physical properties
Dark olive-brown, smooth and subrounded from abrasion, with dull-to-faintly waxy luster and no visible crystals; estimated Mohs hardness 5–6.5, massive texture, and negligible cleavage. The brown-yellow surface coloration likely reflects weathering or iron-oxide staining.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, then became rounded by river, beach, or glacial transport. Its exact geological age cannot be determined from the photograph; basaltic cobbles range from recent to billions of years old.
Uses & applications
Common basalt is used as crushed aggregate, roadstone, riprap, ballast, and occasionally decorative stone. This individual cobble has modest collecting value unless locality or unusual texture is documented.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly has a fine-grained, nonporphyritic appearance. A streak, magnetism, and fresh broken surface would help distinguish it from other dark volcanic rocks.
Field identification & locations
Likely found in volcanic terrains, river gravels, beaches, or glacial deposits. Test for a gray-black fresh interior, weak magnetism, and hardness sufficient to scratch glass; identification remains provisional from the image alone.
