igneous
Rounded gray stone, likely basalt or andesite
Fine-grained mafic to intermediate volcanic rock; likely basalt (plagioclase ± pyroxene, possibly olivine)
AI-generated identification · may be incorrectDark gray, dull to slightly subvitreous, fine-grained and compact, with weathered lighter patches; hardness roughly 6–7 Mohs, no visible cleavage, and typically specific gravity 2.7–3.1.
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Physical properties
Dark gray, dull to slightly subvitreous, fine-grained and compact, with weathered lighter patches; hardness roughly 6–7 Mohs, no visible cleavage, and typically specific gravity 2.7–3.1.
Formation & geological history
Formed when lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; its rounded shape indicates transport and abrasion by water or sediment.
Uses & applications
Used as aggregate, roadstone, riprap, and occasionally building stone; ordinary specimens have little jewelry value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. The image alone cannot reliably distinguish basalt from andesite or weathered fine-grained metamorphic rock.
Field identification & locations
Identify by its uniform fine texture, dark color, heavy feel, and lack of large crystals; a streak, magnet test, acid test, and thin section would improve identification. Common in volcanic regions, riverbeds, beaches, and glacial deposits.
