igneous
Basalt pebble with vesicles/secondary mineral coating
Basalt; predominantly plagioclase, pyroxene, and commonly olivine, with pale amygdules likely zeolite or calcite
AI-generated identification · may be incorrectFine-grained, dark gray-brown, dull to weakly lustrous, with small pale cavities or mineral-filled vesicles; hardness about 5–6 Mohs, no visible cleavage, typical specific gravity 2.8–3.0.
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Physical properties
Fine-grained, dark gray-brown, dull to weakly lustrous, with small pale cavities or mineral-filled vesicles; hardness about 5–6 Mohs, no visible cleavage, typical specific gravity 2.8–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in volcanic regions and often during Cenozoic or older eruptions; exact age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and landscaping material; attractive polished pieces are inexpensive collectors’ specimens.
Geological facts
The rounded shape indicates transport and abrasion in a river, beach, or glacial environment. Pale patches may be amygdules—gas bubbles later filled by minerals.
Field identification & locations
Identify by its dark, very fine-grained texture, high density, and possible weak magnetism from iron-bearing minerals; a streak, hardness, and acid test can help distinguish it from slag or other dark rocks. Common worldwide in volcanic terrains.
