igneous
Fine-grained gray pebble, likely basalt or andesite
Aphanitic volcanic rock; commonly basalt (plagioclase ± pyroxene, often minor olivine)
AI-generated identification · may be incorrectMedium-dark gray, rounded and naturally or water-tumbled, with a dull to slightly silky granular luster and no visible crystals. Typical hardness is about 6–7 Mohs; massive, nonfoliated, with no obvious cleavage; density roughly 2.7–3.0 g/cm³.
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Physical properties
Medium-dark gray, rounded and naturally or water-tumbled, with a dull to slightly silky granular luster and no visible crystals. Typical hardness is about 6–7 Mohs; massive, nonfoliated, with no obvious cleavage; density roughly 2.7–3.0 g/cm³.
Formation & geological history
Likely formed when mafic to intermediate lava cooled rapidly at or near Earth’s surface, producing microscopic crystals; the specimen was later rounded by stream, beach, or commercial tumbling. Its specific geological age and exact composition cannot be determined from the photograph.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping material, and occasionally as a decorative pebble or lapidary specimen. Common examples have little commercial value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly contains tiny plagioclase and pyroxene crystals. A magnet may show weak attraction if iron-bearing minerals are present.
Field identification & locations
Common in volcanic regions, oceanic islands, river gravels, and beaches; identify more confidently by its fine texture, dark gray color, high density, and lack of layering or visible grains. A streak, acid test, magnet test, and thin section would help distinguish basalt from andesite or engineered stone.
