igneous
Weathered vesicular basalt pebble
Basalt, typically plagioclase + pyroxene ± olivine; vesicles may be infilled with secondary minerals
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to subtly glassy, with rounded weathering pits/vesicles. Hardness about 5–6 Mohs; no obvious cleavage; specific gravity commonly 2.8–3.0.
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Physical properties
Dark gray-brown, fine-grained, dull to subtly glassy, with rounded weathering pits/vesicles. Hardness about 5–6 Mohs; no obvious cleavage; specific gravity commonly 2.8–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. The rounded shape and surface pits indicate stream, beach, or general weathering.
Uses & applications
Used as aggregate, roadstone, railway ballast, riprap, and occasionally decorative stone; this small weathered specimen has little jewelry or commercial value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Vesicles are gas bubbles trapped during lava cooling.
Field identification & locations
Identify it by its dark color, fine texture, substantial weight, and lack of visible large crystals; a streak test is usually gray, and it should not fizz with dilute acid. Common in volcanic regions, river gravels, and beaches.
