igneous
Dark rounded pebble, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectThe specimen appears smooth, rounded, dark gray to black, and dull to faintly glassy, consistent with a water-worn basalt pebble. Hardness is approximately 6–6.5 Mohs; crystals are microscopic, with no visible cleavage and typical specific gravity of 2.8–3.0.
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Physical properties
The specimen appears smooth, rounded, dark gray to black, and dull to faintly glassy, consistent with a water-worn basalt pebble. Hardness is approximately 6–6.5 Mohs; crystals are microscopic, with no visible cleavage and typical specific gravity of 2.8–3.0.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its age may range from recent eruptions to Precambrian rocks; the image alone cannot establish age.
Uses & applications
Used as aggregate, road stone, riprap, ballast, and occasionally decorative stone; small rounded pieces are mainly collected or used in crafts.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, tiny holes, or visible olivine grains may occur but are not evident here.
Field identification & locations
Identify it by its uniformly dark, dense appearance, fine texture, and lack of obvious layering; a streak, hardness, and fresh fracture help distinguish it from dark sedimentary rocks. Common in volcanic regions, river gravels, beaches, and glacial deposits.
