igneous
Weathered vesicular basalt
Basalt, primarily plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to weakly metallic-looking surface with small weathering pits; typically Mohs 6, massive, with no obvious cleavage and specific gravity about 2.8–3.0.
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Physical properties
Dark gray-brown, fine-grained, dull to weakly metallic-looking surface with small weathering pits; typically Mohs 6, massive, with no obvious cleavage and specific gravity about 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 Cenozoic or older volcanic activity; the specimen is strongly weathered.
Uses & applications
Used widely as crushed aggregate, roadstone, riprap, railroad ballast, and building stone; attractive polished pieces are occasionally collected, but this specimen has little gem value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust, including mid-ocean ridges and many volcanic islands.
Field identification & locations
Identify it by its dense dark color, fine texture, heft, and lack of visible large crystals; a streak, hardness, and acid test can help distinguish it from weathered slag or other dark rocks. Common locations include volcanic regions worldwide.
