igneous
Rounded, porphyritic volcanic pebble (likely andesite)
Porphyritic andesite; exact identification requires examination or testing
AI-generated identification · may be incorrectDark gray-green, fine-grained groundmass with visible pale feldspar-like grains and a small reddish-brown inclusion. Likely hardness about 6 on Mohs scale; dull to slightly lustrous, with no obvious cleavage and a dense, compact texture.
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Physical properties
Dark gray-green, fine-grained groundmass with visible pale feldspar-like grains and a small reddish-brown inclusion. Likely hardness about 6 on Mohs scale; dull to slightly lustrous, with no obvious cleavage and a dense, compact texture.
Formation & geological history
Likely formed when magma cooled, with larger crystals growing before the remaining melt solidified rapidly; later weathering and water transport rounded it. Its age and precise formation environment cannot be determined from the photograph.
Uses & applications
Primarily a natural pebble or collectible; andesite is also used locally as crushed aggregate and building stone.
Geological facts
The contrasting larger grains are called phenocrysts, a texture that records two stages of cooling.
Field identification & locations
Look for a fine-grained volcanic matrix containing distinct crystals and a rounded, water-worn surface. Similar rocks occur in volcanic regions and river or beach gravels; confirm with a fresh break, hardness, and mineral tests.
