igneous
Tumbled dark gray stone, possibly basalt or andesite
Fine-grained mafic to intermediate volcanic rock; exact mineralogy cannot be confirmed from the photograph
AI-generated identification · may be incorrectHardness about 5.5–6.5 Mohs; dark gray with pale weathering spots, dull to weakly vitreous luster, and no visible crystals or cleavage. Likely composed mainly of plagioclase, pyroxene, and possibly magnetite.
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Physical properties
Hardness about 5.5–6.5 Mohs; dark gray with pale weathering spots, dull to weakly vitreous luster, and no visible crystals or cleavage. Likely composed mainly of plagioclase, pyroxene, and possibly magnetite.
Formation & geological history
Probably formed when volcanic lava cooled rapidly at or near Earth’s surface; such rocks range from young lava flows to Precambrian material. The rounded shape appears human-tumbled or naturally water-worn.
Uses & applications
Used as aggregate, road stone, landscaping material, and occasionally as a low-cost polished specimen; generally has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust, while andesite commonly forms at volcanic arcs. A magnet may attract it weakly if magnetite is present.
Field identification & locations
Look for a fine-grained, heavy, nonfoliated texture and a dark streak or pale gray interior; a streak, magnet, and hardness test help distinguish it from slag or soft sedimentary rock. Common worldwide, especially volcanic regions and stream gravels.
