igneous
Vesicular basalt (weathered river stone)
Basalt, chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to subtly waxy luster, with irregular vesicles and weathering cavities; hardness about 5–6 Mohs, massive structure, generally no visible cleavage. The rounded shape indicates prolonged water abrasion or tumbling.
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Physical properties
Dark gray-brown, fine-grained, dull to subtly waxy luster, with irregular vesicles and weathering cavities; hardness about 5–6 Mohs, massive structure, generally no visible cleavage. The rounded shape indicates prolonged water abrasion or tumbling.
Formation & geological history
An extrusive mafic igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Gas bubbles produced the original vesicles, later enlarged by weathering and erosion.
Uses & applications
Used as aggregate, road stone, riprap, landscaping material, and occasionally as a carving or decorative stone; this ordinary specimen has minimal gem value.
Geological facts
Vesicles distinguish it from most dense gabbro, while its fine texture distinguishes it from coarse-grained intrusive rocks. Brown coloration likely reflects oxidation of iron-bearing minerals.
Field identification & locations
Common in volcanic regions and riverbeds worldwide; identify by its dark fine-grained matrix, low density relative to granite, and cavities. A steel nail typically does not scratch fresh basalt.
