igneous
Weathered dark volcanic rock, likely basalt
Basalt; fine-grained mafic silicate rock dominated by plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to brown, dull to slightly earthy luster, very fine-grained and massive; Mohs hardness about 6, with no obvious cleavage. The brown coloration likely reflects surface weathering and iron oxidation.
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Physical properties
Typically dark gray to brown, dull to slightly earthy luster, very fine-grained and massive; Mohs hardness about 6, with no obvious cleavage. The brown coloration likely reflects surface weathering and iron oxidation.
Formation & geological history
Forms when basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows, oceanic crust, and flood-basalt provinces; specimens may range from modern to Precambrian in age.
Uses & applications
Widely used as crushed aggregate, road stone, railway ballast, riprap, and building stone; attractive polished varieties are used decoratively but ordinary fragments have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its identification is provisional from the photograph because grain size and mineral texture are poorly resolved.
Field identification & locations
Look for a dense, fine-grained dark rock that is harder than steel and lacks visible quartz or large crystals; a streak, magnetism test, or fresh broken surface helps distinguish it from weathered sedimentary rocks. Common worldwide, especially volcanic regions and oceanic crust.
