igneous
Porphyritic basalt (likely amygdaloidal basalt)
Basalt, a mafic volcanic rock; composition typically plagioclase + pyroxene, with possible olivine and vesicle-filling minerals
AI-generated identification · may be incorrectFine dark gray matrix containing conspicuous white plagioclase phenocrysts and black mafic crystals; rounded cavities suggest vesicles, some mineral-filled. Hardness about 5–6, dull to slightly glassy luster, very fine-grained groundmass, no prominent cleavage, specific gravity roughly 2.8–3.1.
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Physical properties
Fine dark gray matrix containing conspicuous white plagioclase phenocrysts and black mafic crystals; rounded cavities suggest vesicles, some mineral-filled. Hardness about 5–6, dull to slightly glassy luster, very fine-grained groundmass, no prominent cleavage, specific gravity roughly 2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled relatively quickly at or near Earth’s surface, trapping gas bubbles and larger early-grown crystals. Most examples are Cenozoic or younger, though basalt occurs throughout geologic time.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and building stone; attractive polished pieces are collected but generally have little gem value.
Geological facts
The contrasting larger crystals give it a porphyritic texture; cavities may later fill with quartz, calcite, zeolites, or other secondary minerals.
Field identification & locations
Common in volcanic regions, including Hawaii, Iceland, the Pacific Northwest, India’s Deccan Traps, and oceanic crust. A dark fine-grained matrix with white feldspar crystals and vesicles distinguishes it from granite and many sedimentary rocks.
