igneous
Porphyritic basalt (likely vesicular basalt)
Basalt; predominantly plagioclase feldspar and pyroxene, with possible olivine and secondary zeolite/calcite
AI-generated identification · may be incorrectDark gray to black, fine-grained groundmass with conspicuous pale mineral grains and irregular vesicles or cavities; typically vitreous to dull luster, hardness about 5–6, massive texture, no obvious cleavage, specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray to black, fine-grained groundmass with conspicuous pale mineral grains and irregular vesicles or cavities; typically vitreous to dull luster, hardness about 5–6, massive texture, no obvious cleavage, specific gravity roughly 2.8–3.1.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows; the specimen’s age cannot be determined from an image and may range from recent to hundreds of millions of years.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and building stone; attractive polished pieces are sold as decorative specimens, though this irregular fragment has modest commercial value.
Geological facts
The light patches may be feldspar or filled vesicles, while rounded dark grains could be olivine or weathered inclusions; a definite identification requires a fresh surface, hand lens, streak, and hardness tests.
Field identification & locations
Common in volcanic regions, oceanic crust, Iceland, Hawaii, India’s Deccan Traps, and many continental lava fields. It is identified by its dark mafic color, heavy feel, fine texture, and occasional gas cavities or pale feldspar crystals.
