igneous
Dark basaltic rock fragment
Basalt, likely vesicular or weathered basalt; chiefly plagioclase, pyroxene, and possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, fine-grained, and dull to weakly vitreous; often massive, with little visible cleavage. The specimen appears dense and irregular, with possible weathering or minor vesicles.
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Physical properties
Typically Mohs 5–6, dark gray to black, fine-grained, and dull to weakly vitreous; often massive, with little visible cleavage. The specimen appears dense and irregular, with possible weathering or minor vesicles.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly at volcanic fields, mid-ocean ridges, and hotspots. Its age cannot be determined from the photograph; it may be recent or millions to billions of years old.
Uses & applications
Used widely as crushed aggregate, road base, ballast, riprap, and building stone; attractive polished pieces are collected but it is rarely used as a gemstone.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Magnetic grains may make some basalt weakly magnetic.
Field identification & locations
Identify it by its black fine-grained appearance, high density, and lack of obvious individual crystals; distinguish it from coal by its mineral texture and lack of sooty streak. Common worldwide, especially volcanic regions.
