igneous
Polished basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, smooth and rounded from water or tumbling, with a dull to subtly vitreous luster. Hardness is about 6–7 Mohs; typically massive, fine-grained, noncleaved, and relatively dense (specific gravity ~2.8–3.0).
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Physical properties
Dark gray to black, smooth and rounded from water or tumbling, with a dull to subtly vitreous luster. Hardness is about 6–7 Mohs; typically massive, fine-grained, noncleaved, and relatively dense (specific gravity ~2.8–3.0).
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and ancient lava flows. Its age may range from recent to billions of years; the image alone cannot establish age.
Uses & applications
Used as aggregate, roadstone, riprap, paving stone, ballast, and landscaping material; polished pebbles are sold for crafts and collections.
Geological facts
The specimen’s rounded, uniformly dark appearance is consistent with basalt, though fine-grained andesite or another dark volcanic rock is possible without testing.
Field identification & locations
Commonly found in volcanic regions, riverbeds, beaches, and glacial deposits. Confirm with a hand lens, high density, lack of layering, and usually weak reaction to dilute hydrochloric acid; a streak test is generally inconclusive.
