igneous
Weathered basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dull to slightly vitreous, Mohs hardness about 6; dense, with no visible cleavage and commonly vesicular or massive. The specimen appears strongly weathered, with pale alteration coatings and rusty iron staining.
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Physical properties
Typically dark gray to black, fine-grained, dull to slightly vitreous, Mohs hardness about 6; dense, with no visible cleavage and commonly vesicular or massive. The specimen appears strongly weathered, with pale alteration coatings and rusty iron staining.
Formation & geological history
Forms when basaltic lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. Its exact age cannot be determined from the image; weathering reflects later exposure to water and oxygen.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished pieces are collected but it is rarely a jewelry gemstone.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Weathering can convert its iron-bearing minerals to reddish-brown oxides and clay minerals.
Field identification & locations
Identify by its heavy, dark, fine texture and lack of obvious large crystals; a streak, density, and fresh broken surface help distinguish it from weathered gabbro or slag. Common worldwide, especially volcanic islands and oceanic regions.
