igneous
Black basalt fragment
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, dull to weakly vitreous, and fine-grained with microscopic interlocking crystals; the specimen may show weathering or slight vesicles. Cleavage is not visible at the rock scale; specific gravity is about 2.8–3.0.
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Physical properties
Typically Mohs 5–6, dark gray to black, dull to weakly vitreous, and fine-grained with microscopic interlocking crystals; the specimen may show weathering or slight vesicles. Cleavage is not visible at the rock scale; specific gravity is about 2.8–3.0.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows, dikes, oceanic crust, or flood-basalt provinces. Its exact age cannot be determined from the image and may range from recent to Precambrian.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, dimension stone, and occasionally ornamental stone. Unusual polished pieces are collected, but ordinary fragments have minimal commercial value.
Geological facts
Basalt is the most common volcanic rock and makes up much of the oceanic crust. Magnetic minerals may cause a noticeable response to a strong magnet.
Field identification & locations
Common worldwide, especially around volcanic regions and ocean floors; identify by its uniform dark color, fine texture, heft, and lack of obvious large crystals. A hand lens, streak test, magnet test, and acid test can help distinguish it from slag or other dark rocks.
