igneous
Rounded basalt pebble
Fine-grained mafic volcanic rock, commonly basalt (plagioclase + pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray, smooth and subrounded from transport; likely Mohs hardness 5–6, dull to slightly vitreous luster, very fine-grained interlocking crystals, and no visible cleavage. Specific gravity is typically about 2.8–3.0.
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Physical properties
Dark gray, smooth and subrounded from transport; likely Mohs hardness 5–6, dull to slightly vitreous luster, very fine-grained interlocking crystals, and no visible cleavage. Specific gravity is typically about 2.8–3.0.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows; the pebble’s rounded shape indicates stream, beach, or glacial abrasion. Basalt ranges from Archean to modern age, so its age cannot be determined from the image.
Uses & applications
Commonly used as crushed aggregate, road base, riprap, railway ballast, and paving stone; attractive polished pieces are occasionally collected but usually have low commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of oceanic crust. The specimen could also be a closely related fine-grained rock such as andesite, which requires testing or microscopy to distinguish confidently.
Field identification & locations
Identify by its dark color, heavy feel, fine texture, and lack of obvious layering or large crystals; a streak test, magnetism test, and fresh broken surface help. Common worldwide, especially near volcanic terrains and stream gravels.
