igneous
Weathered dark volcanic rock, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray-brown, dull to weakly vitreous luster, dense and fine-grained, with irregular fracture and possible vesicles or weathering cavities; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The orange-brown patches are likely iron-oxide weathering.
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Physical properties
Dark gray-brown, dull to weakly vitreous luster, dense and fine-grained, with irregular fracture and possible vesicles or weathering cavities; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The orange-brown patches are likely iron-oxide weathering.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic fields; specimens may range from recent to Precambrian in age.
Uses & applications
Crushed basalt is used for aggregate, roadstone, railroad ballast, and concrete; attractive polished pieces are used decoratively, but this specimen has little gem value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals.
Field identification & locations
Identify by its dark, heavy, compact appearance and lack of visible large crystals; a streak, magnetism test, and fresh break help distinguish it from slag or metamorphic rocks. Common worldwide in volcanic regions.
