igneous
Black pebble, likely basalt
Basalt, fine-grained mafic volcanic rock; possibly weathered basaltic glass
AI-generated identification · may be incorrectDark black, fine-grained, and weakly to moderately vitreous with rounded weathering surfaces; typically Mohs 5–6, specific gravity about 2.8–3.0, composed mainly of plagioclase, pyroxene, and sometimes olivine. The image is insufficient to confirm mineral grains or vesicles.
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Physical properties
Dark black, fine-grained, and weakly to moderately vitreous with rounded weathering surfaces; typically Mohs 5–6, specific gravity about 2.8–3.0, composed mainly of plagioclase, pyroxene, and sometimes olivine. The image is insufficient to confirm mineral grains or vesicles.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and river gravels; specimens may range from Precambrian to modern. Its rounded shape suggests transport and abrasion in water or sediment.
Uses & applications
Used extensively as crushed aggregate, road material, riprap, railroad ballast, and dimension stone; attractive polished pieces are sold as inexpensive decorative specimens. This small, unverified pebble has little commercial value.
Geological facts
Black volcanic pebbles can also be andesite, chert, obsidian, or industrial slag, which cannot be reliably distinguished from this photograph alone.
Field identification & locations
Test for a consistent fine texture, moderate density, and hardness sufficient to scratch glass; basalt usually does not fizz in dilute acid and is nonmagnetic or only weakly magnetic. Common in volcanic regions and stream beaches.
