igneous
Porphyritic basalt (likely vesicular basalt)
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase feldspar and pyroxene, with possible olivine phenocrysts
AI-generated identification · may be incorrectDark gray to black, fine-grained matrix with conspicuous pale cream feldspar and orange-brown olivine crystals; generally dull to slightly glassy, hardness about 5–6, massive with no obvious cleavage. Rounded shape suggests stream transport or human tumbling.
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Physical properties
Dark gray to black, fine-grained matrix with conspicuous pale cream feldspar and orange-brown olivine crystals; generally dull to slightly glassy, hardness about 5–6, massive with no obvious cleavage. Rounded shape suggests stream transport or human tumbling.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes; the visible crystals crystallized before eruption. Exact age cannot be determined from the photograph.
Uses & applications
Commonly used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces may be sold as display specimens, though this example appears ordinary.
Geological facts
The pale crystals are likely plagioclase, while orange grains may be weathered olivine; the texture is termed porphyritic. Vesicles may indicate gas-rich lava, although none are clearly visible here.
Field identification & locations
Identify it by its dark, dense volcanic matrix and scattered light feldspar or green-orange olivine crystals; it is common in volcanic regions and stream gravels worldwide. A hand lens and streak, hardness, and density tests would improve confidence.
