igneous
Rounded river pebble, likely basaltic rock
Fine-grained mafic igneous rock, probably basalt or andesite; exact identification requires testing
AI-generated identification · may be incorrectDark brown-gray, smooth and subrounded from abrasion, dull to slightly earthy luster, and apparently massive with no visible cleavage. Likely hardness about 5–6 Mohs and specific gravity around 2.7–3.1; fine grains are not resolvable in the image.
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Physical properties
Dark brown-gray, smooth and subrounded from abrasion, dull to slightly earthy luster, and apparently massive with no visible cleavage. Likely hardness about 5–6 Mohs and specific gravity around 2.7–3.1; fine grains are not resolvable in the image.
Formation & geological history
Probably formed when mafic lava cooled rapidly at or near Earth’s surface, then was transported and rounded by water; its precise geological age is unknown. Common in volcanic terrains and stream or beach gravels.
Uses & applications
Typically used as landscaping stone, aggregate, drainage material, or a hand specimen; this individual pebble has negligible commercial value.
Geological facts
The rounded shape indicates prolonged abrasion rather than crystal growth. Brown coloration may reflect surface weathering and oxidation of iron-bearing minerals.
Field identification & locations
Look for a dense, fine-grained texture, weak magnetism, and a dark streak-free interior when freshly broken; vinegar should not fizz. Common in riverbeds, beaches, and glacial deposits; a geological hammer, streak test, and magnet improve identification.
