igneous
Porphyritic basalt
Vesicular/porphyritic basalt, chiefly plagioclase and pyroxene with iron-oxide alteration
AI-generated identification · may be incorrectDark fine-grained volcanic matrix with rounded reddish-brown inclusions, likely oxidized olivine or amygdales; hardness about 5–6, dull to faintly vitreous luster, massive texture, no obvious cleavage, specific gravity roughly 2.8–3.1.
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Physical properties
Dark fine-grained volcanic matrix with rounded reddish-brown inclusions, likely oxidized olivine or amygdales; hardness about 5–6, dull to faintly vitreous luster, massive texture, no obvious cleavage, specific gravity roughly 2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping larger crystals or later filling vesicles with secondary minerals; typically Cenozoic or younger, though basalt occurs throughout geologic time.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, and dimension stone; attractive polished pieces are collected, but this small specimen has modest commercial value.
Geological facts
The reddish spots may be weathered olivine phenocrysts, iron-rich mineral patches, or mineral-filled gas cavities (amygdales); a precise identification requires a fresh surface or testing.
Field identification & locations
Common in volcanic regions such as Hawaii, Iceland, India’s Deccan Traps, and the Pacific Northwest. Field clues include black dense rock, fine grain, occasional vesicles, and magnetic response; collector value is usually $2–$15.
