igneous
Rounded dark river pebble (likely basalt or fine-grained andesite)
Probable basalt; plagioclase and pyroxene, with minor olivine or iron oxides possible
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; Mohs hardness about 5–6, specific gravity roughly 2.8–3.1. Its smooth, rounded form reflects abrasion in water or sediment.
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Physical properties
Dark gray-brown, fine-grained, dull to faintly satin luster, massive with no visible crystals or cleavage; Mohs hardness about 5–6, specific gravity roughly 2.8–3.1. Its smooth, rounded form reflects abrasion in water or sediment.
Formation & geological history
Likely an igneous volcanic rock formed when mafic lava cooled rapidly, commonly in the Cenozoic or older; exact age and identity cannot be determined from the photograph alone. Rounding indicates transport in a river, beach, or glacial deposit.
Uses & applications
Useful mainly as a natural pebble, landscaping stone, aggregate, or teaching specimen; generally little jewelry or collector value unless unusually patterned or locally significant.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. A magnet may attract some specimens because of magnetite.
Field identification & locations
Look for a uniformly fine texture, dark color, substantial weight, and absence of fizz in dilute acid; a fresh broken surface is more diagnostic than the weathered exterior. Common worldwide in volcanic regions and river gravels.
