igneous
Porphyritic basalt or andesite pebble
Porphyritic volcanic rock; commonly plagioclase phenocrysts in a fine mafic-to-intermediate matrix
AI-generated identification · may be incorrectGray-green, rounded waterworn pebble with abundant pale, blocky crystals in a darker fine-grained groundmass; likely hardness about 6–7, dull to slightly vitreous luster, dense, with no obvious cleavage. The pale crystals are probably feldspar, possibly with altered mafic minerals.
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Physical properties
Gray-green, rounded waterworn pebble with abundant pale, blocky crystals in a darker fine-grained groundmass; likely hardness about 6–7, dull to slightly vitreous luster, dense, with no obvious cleavage. The pale crystals are probably feldspar, possibly with altered mafic minerals.
Formation & geological history
Formed when volcanic lava cooled rapidly at or near Earth’s surface, preserving larger crystals that crystallized earlier; exact age cannot be determined from the image. Such rocks commonly occur in volcanic terrains and river or beach gravels.
Uses & applications
Used locally as durable aggregate, landscaping stone, road material, and occasional decorative specimens; generally not a jewelry stone.
Geological facts
Its conspicuous large crystals are called phenocrysts, while the finer surrounding material is the groundmass—a classic porphyritic texture.
Field identification & locations
Identify by its dense volcanic texture, feldspar-like white crystals, and lack of sedimentary layering or foliation; a streak, acid test, and thin section would distinguish basalt from andesite. Common in volcanic regions and downstream gravels.
