igneous
Rounded dark pebble, likely basalt or volcanic rock
Fine-grained mafic volcanic rock; commonly basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray-brown, dull to slightly satin luster, fine-grained and massive, with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The rounded shape and surface wear indicate natural abrasion or tumbling.
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Physical properties
Dark gray-brown, dull to slightly satin luster, fine-grained and massive, with no visible crystals or cleavage; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.1. The rounded shape and surface wear indicate natural abrasion or tumbling.
Formation & geological history
Likely formed when mafic lava rapidly cooled at or near Earth’s surface, generally during the Cenozoic or older; its exact age cannot be determined visually. It may have been transported and rounded by a stream, beach, or glacial environment.
Uses & applications
Basalt is widely used as crushed aggregate, road material, riprap, and building stone; attractive polished pieces are used as inexpensive curios or cabochons.
Geological facts
Basalt commonly contains microscopic plagioclase and pyroxene crystals, so it appears uniform without magnification. Vesicles, if present, may indicate gas bubbles in the original lava.
Field identification & locations
Identify it by its dark color, fine texture, heavy feel, lack of obvious layering, and inability to scratch it easily with a steel blade; a streak, magnetism, and acid test could refine identification. Common worldwide in volcanic regions and river gravels.
