igneous
Weathered porphyritic volcanic rock (likely andesite or basalt)
Fine-grained intermediate-to-mafic volcanic rock; likely andesite/basalt composed mainly of plagioclase, pyroxene, and possibly altered iron-bearing minerals
AI-generated identification · may be incorrectBrown-gray, rounded and strongly weathered with pale mineral patches and possible small phenocrysts; dull to slightly earthy luster, hardness about 5–6, massive texture, no obvious cleavage, and typical specific gravity around 2.7–3.0.
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Physical properties
Brown-gray, rounded and strongly weathered with pale mineral patches and possible small phenocrysts; dull to slightly earthy luster, hardness about 5–6, massive texture, no obvious cleavage, and typical specific gravity around 2.7–3.0.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, then underwent oxidation and chemical weathering; its precise geological age and composition cannot be determined from the photograph alone.
Uses & applications
Commonly used as aggregate, road stone, landscaping material, and an occasional collecting specimen; this small weathered piece has little gem or commercial value.
Geological facts
The pale patches may be feldspar or weathering products, while the reddish-brown color commonly reflects oxidation of iron-bearing minerals. A fresh broken surface would reveal whether it is truly basaltic or andesitic.
Field identification & locations
Common in volcanic terrains and stream gravels worldwide. Field tests include checking for a fine-grained crystalline texture, lack of layering, moderate heft, and a streakless or very faint streak; acid testing is generally unhelpful.
