igneous
Dark gray-green river stone, likely basalt or metabasalt
Basalt (fine-grained mafic volcanic rock), possibly weakly metamorphosed to metabasalt
AI-generated identification · may be incorrectTypically 5–6 Mohs; dark gray to greenish-black, fine-grained, dull to subtly waxy luster, with no visible crystals or cleavage. Usually composed mainly of plagioclase and pyroxene, commonly with magnetite and alteration minerals such as chlorite.
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Physical properties
Typically 5–6 Mohs; dark gray to greenish-black, fine-grained, dull to subtly waxy luster, with no visible crystals or cleavage. Usually composed mainly of plagioclase and pyroxene, commonly with magnetite and alteration minerals such as chlorite.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in oceanic crust and volcanic fields; associated rocks range from Precambrian to modern. Its rounded shape indicates transport and abrasion in a river, beach, or glacial deposit.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally as an ornamental or polished stone; ordinary specimens have little jewelry value.
Geological facts
The specimen’s smooth, rounded form and mottled surface are consistent with a weathered mafic cobble, but an image alone cannot distinguish basalt confidently from amphibolite, greenstone, or dark schist.
Field identification & locations
Look for a very fine texture, high density, dark color, and possible weak magnetic response; a streak, thin section, hardness test, and magnetic test would improve identification. Common worldwide, especially in volcanic and glacial regions.
