igneous
Weathered porphyritic volcanic rock, possibly andesite
Porphyritic andesite (tentative; exact identification requires testing)
AI-generated identification · may be incorrectDark gray-brown, fine-grained groundmass with scattered pale crystals and rusty weathering; likely plagioclase phenocrysts. Hardness roughly 5–6, generally dull, with no obvious cleavage; crystal structure is fine-grained and partly crystalline.
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Physical properties
Dark gray-brown, fine-grained groundmass with scattered pale crystals and rusty weathering; likely plagioclase phenocrysts. Hardness roughly 5–6, generally dull, with no obvious cleavage; crystal structure is fine-grained and partly crystalline.
Formation & geological history
Likely formed when intermediate-composition lava cooled, with larger crystals growing before the remaining melt solidified rapidly. Its geological age cannot be determined from the photograph.
Uses & applications
Similar andesite is used as crushed aggregate, paving stone, and building material; this small rounded specimen is mainly of modest collecting interest.
Geological facts
The visible pale flecks are consistent with larger crystals embedded in a finer volcanic matrix, a texture called porphyritic.
Field identification & locations
Look for a fine, compact matrix containing scattered light-colored grains; the rounded shape likely reflects transport or weathering. Andesite is common in volcanic arcs, including the Andes, Japan, and Cascades.
