igneous
Porphyritic basalt (possibly vesicular basalt)
Mafic volcanic rock; mainly plagioclase feldspar and pyroxene, with possible olivine phenocrysts
AI-generated identification · may be incorrectDark gray-black, fine-grained groundmass with numerous pale pink-to-brown rounded phenocrysts; generally dull to slightly vitreous. Typical hardness is about 5–6, with no obvious cleavage at hand-specimen scale and specific gravity around 2.8–3.1.
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Physical properties
Dark gray-black, fine-grained groundmass with numerous pale pink-to-brown rounded phenocrysts; generally dull to slightly vitreous. Typical hardness is about 5–6, with no obvious cleavage at hand-specimen scale and specific gravity around 2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, preserving larger crystals that grew before eruption. Such rocks commonly range from recent volcanic material to Paleozoic and older flows; the exact age cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road metal, railway ballast, riprap, and occasionally decorative stone. Ordinary specimens have little gem or commercial value but can be attractive for teaching and collecting.
Geological facts
The contrasting larger crystals indicate a porphyritic texture, recording two-stage cooling: slower underground crystallization followed by rapid surface cooling.
Field identification & locations
Identify it by its very dark fine-grained matrix, high density, and conspicuous embedded crystals; a streak, magnet test, and fresh break help distinguish basalt from andesite or weathered scoria. Common worldwide in volcanic regions, including Hawaii, Iceland, Japan, the Andes, and oceanic crust.
