igneous
Porphyritic basaltic-andesite cobble
Fine-grained plagioclase–pyroxene volcanic rock, likely basaltic andesite
AI-generated identification · may be incorrectDark gray, rounded, and weakly weathered, with fine crystalline groundmass and scattered lighter grains; typically Mohs 6–7, dull to slightly vitreous luster, no obvious cleavage, and specific gravity about 2.7–2.9.
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Physical properties
Dark gray, rounded, and weakly weathered, with fine crystalline groundmass and scattered lighter grains; typically Mohs 6–7, dull to slightly vitreous luster, no obvious cleavage, and specific gravity about 2.7–2.9.
Formation & geological history
Formed when mafic to intermediate lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic volcanic activity; the rounded shape indicates later stream or beach transport.
Uses & applications
Used as durable aggregate, riprap, road stone, and occasionally as landscaping material; ordinary cobbles have little commercial or jewelry value.
Geological facts
The rusty-brown patches are likely iron-oxide weathering, while the small pale crystals are probably plagioclase feldspar. Its compact texture and lack of layering distinguish it from sedimentary rocks.
Field identification & locations
Look for a dense, dark volcanic texture, hardness sufficient to scratch glass, and weak or absent reaction to dilute acid. Common in volcanic regions worldwide, including the Cascades, Andes, Japan, Iceland, and Mediterranean belts.
