igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with minor olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark gray to black, smooth and well-rounded, with a dull to subtly glassy luster; typically fine-grained, hardness about 6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The surface appears naturally weathered or water-polished.
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Physical properties
Dark gray to black, smooth and well-rounded, with a dull to subtly glassy luster; typically fine-grained, hardness about 6 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.0. The surface appears naturally weathered or water-polished.
Formation & geological history
Basalt forms when low-silica magma rapidly cools at or near Earth’s surface, commonly in lava flows, oceanic crust, and volcanic islands. The pebble was likely rounded by stream, beach, or glacial transport; its exact geological age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and occasionally decorative stone. Ordinary pebbles have little jewelry or collector value, though polished basalt can be sold as décor.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may indicate gas bubbles in the original lava, but none are clearly visible here.
Field identification & locations
Identify by its dark color, fine texture, heavy feel, and lack of obvious crystals; a streak, hardness, and acid test can help distinguish it from slate, chert, or industrial slag. Common worldwide in volcanic and alluvial settings.
