igneous
Weathered vesicular basalt (likely amygdaloidal basalt)
Basalt; chiefly plagioclase, pyroxene, and minor olivine/iron oxides, with possible secondary zeolite or calcite fillings
AI-generated identification · may be incorrectFine-grained, dark-to-brown-gray volcanic rock with abundant rounded vesicles, weathering rind, and dull earthy luster; hardness about 5–6, no obvious cleavage, typically dense except where porous. The exposed cavities and mottled alteration are characteristic of weathered basaltic lava.
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Physical properties
Fine-grained, dark-to-brown-gray volcanic rock with abundant rounded vesicles, weathering rind, and dull earthy luster; hardness about 5–6, no obvious cleavage, typically dense except where porous. The exposed cavities and mottled alteration are characteristic of weathered basaltic lava.
Formation & geological history
Formed when low-silica lava cooled rapidly at or near Earth’s surface, trapping gas bubbles; some cavities may later have filled with minerals. Basalt commonly formed from the Precambrian to Recent, depending on locality.
Uses & applications
Used as aggregate, road stone, riprap, landscaping rock, and occasionally as a decorative material; this weathered cobble has little gem value.
Geological facts
Vesicles record gas exsolution during eruption, while reddish-brown surfaces reflect oxidation of iron-bearing minerals. Rounded shape indicates transport and abrasion in a river, beach, or glacial deposit.
Field identification & locations
Identify by its fine texture, dark mafic composition, iron staining, and abundant gas cavities; distinguish from pumice because it is much heavier and less glassy. Common worldwide in volcanic terrains and transported gravel beds.
