igneous
Porphyritic volcanic rock (likely andesite or basaltic andesite)
Porphyritic andesite/basaltic andesite; mainly plagioclase, pyroxene, and possibly olivine in a fine volcanic groundmass
AI-generated identification · may be incorrectHardness about 5.5–6.5; rounded and weathered, with a dull to slightly vitreous surface. Dark gray-black crystals and rusty tan/orange altered minerals are set in a fine beige-gray matrix; crystals are porphyritic, with no obvious cleavage at specimen scale.
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Physical properties
Hardness about 5.5–6.5; rounded and weathered, with a dull to slightly vitreous surface. Dark gray-black crystals and rusty tan/orange altered minerals are set in a fine beige-gray matrix; crystals are porphyritic, with no obvious cleavage at specimen scale.
Formation & geological history
Formed when magma cooled in two stages: larger crystals grew underground, followed by rapid cooling during eruption. The exact geological age cannot be determined from the image; such rocks form in volcanic arcs, rift zones, and oceanic-island settings.
Uses & applications
Commonly used as aggregate, road stone, and landscaping rock; attractive pieces may be collected, though it is generally not a jewelry gemstone.
Geological facts
The rounded shape indicates transport and abrasion, likely as a stream or beach cobble. Orange material may represent oxidized iron-bearing minerals, while dark grains are likely pyroxene or altered mafic minerals.
Field identification & locations
Identify it by its fine-grained volcanic matrix, scattered visible dark phenocrysts, and lack of sedimentary layering or metamorphic foliation; a thin section or chemical test is needed to distinguish andesite from basalt. Common in volcanic terrains worldwide.
