igneous
Porphyritic basalt (likely vesicular basalt)
Basalt, chiefly plagioclase feldspar and pyroxene, with possible olivine phenocrysts
AI-generated identification · may be incorrectDark gray-black, fine-grained groundmass with conspicuous salmon-pink to tan crystals or weathered inclusions; typically dull to subvitreous, hardness about 5–6, massive and non-cleavable. The rounded shape is from stream or beach abrasion; the pink grains may be feldspar or altered minerals.
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Physical properties
Dark gray-black, fine-grained groundmass with conspicuous salmon-pink to tan crystals or weathered inclusions; typically dull to subvitreous, hardness about 5–6, massive and non-cleavable. The rounded shape is from stream or beach abrasion; the pink grains may be feldspar or altered minerals.
Formation & geological history
An extrusive volcanic rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. The surrounding rounded gravel suggests transport by water or waves.
Uses & applications
Used as crushed aggregate, road stone, riprap, and building stone; attractive polished pieces may be collected, but it is generally not a gemstone.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly contains small olivine, pyroxene, and plagioclase crystals. A streak, acid test, and close inspection with a hand lens can distinguish it from slag or cemented gravel.
Field identification & locations
Common worldwide in volcanic regions, including Hawaii, Iceland, the Pacific Northwest, and oceanic islands. Identify it by its dense dark matrix, scattered crystalline grains, lack of obvious layering, and nonmetallic appearance.
