igneous
Dark weathered stone, likely basalt
Basalt (fine-grained mafic volcanic rock; commonly plagioclase, pyroxene, and minor olivine)
AI-generated identification · may be incorrectHardness about 5–6 Mohs; dark gray to brown, dull to weakly vitreous luster, massive fine-grained texture, and no visible cleavage. The rounded shape and surface patina indicate substantial abrasion and weathering.
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Physical properties
Hardness about 5–6 Mohs; dark gray to brown, dull to weakly vitreous luster, massive fine-grained texture, and no visible cleavage. The rounded shape and surface patina indicate substantial abrasion and weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Individual formation ages vary from recent eruptions to Precambrian rocks.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, building stone, and sometimes landscaping material. Ordinary specimens have little jewelry value, though polished basalt can be decorative.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Identify it by its dark, dense, fine-grained, non-layered appearance and resistance to scratching; a steel file may scratch some weathered surfaces. Common worldwide, especially near volcanic regions; the image alone cannot distinguish basalt confidently from dark andesite or another weathered stone.
