igneous
Likely weathered basalt or mafic volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly glassy luster, with pale weathering or alteration. Basalt typically has Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.0; the image is somewhat blurred, so exact identification is uncertain.
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Physical properties
Dark gray to black, fine-grained, dull to weakly glassy luster, with pale weathering or alteration. Basalt typically has Mohs hardness 5–6, no obvious cleavage, and specific gravity about 2.8–3.0; the image is somewhat blurred, so exact identification is uncertain.
Formation & geological history
Forms when mafic lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Basalt ranges from ancient Precambrian flows to modern eruptions and may develop lighter alteration rims through weathering.
Uses & applications
Commonly used as crushed aggregate, road metal, riprap, railroad ballast, and building stone; attractive polished pieces are occasionally collected. This irregular specimen has limited commercial value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and commonly forms broad lava plateaus and the ocean floor. Vesicles, crystals of olivine, or magnetic behavior can help distinguish it from other dark rocks.
Field identification & locations
Test for a fine, interlocking texture, dark streak, substantial density, and weak attraction to a magnet if magnetite is present; a fresh broken surface should be dark gray. Common locations include Hawaii, Iceland, India’s Deccan Traps, and oceanic crust.
