igneous
Weathered basaltic rock
Basalt, likely amygdaloidal or altered basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray to brown, fine-grained, massive, and dull to weakly vitreous; hardness about 5–6 Mohs, with no obvious cleavage. Brown surfaces likely represent iron-oxide weathering; pale patches may be alteration minerals or embedded feldspar.
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Physical properties
Dark gray to brown, fine-grained, massive, and dull to weakly vitreous; hardness about 5–6 Mohs, with no obvious cleavage. Brown surfaces likely represent iron-oxide weathering; pale patches may be alteration minerals or embedded feldspar.
Formation & geological history
Basalt forms when low-silica basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic flows or oceanic crust. Its exact age and alteration history cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, roadstone, riprap, railway ballast, and occasionally building stone; unremarkable pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and commonly contains calcium-rich plagioclase and pyroxene. Vesicles, if present beneath the weathered surface, may later be filled by zeolites, calcite, or quartz.
Field identification & locations
Identify it by its dark, fine-grained texture, substantial weight, and resistance to scratching; a glassy streak or abundant quartz would suggest another rock. Common locations include volcanic regions, oceanic islands, and old lava fields.
