igneous
Weathered basalt or andesite cobble
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase ± pyroxene, amphibole, and iron oxides
AI-generated identification · may be incorrectDark gray-green, dull to slightly waxy surface with rounded weathering; likely hardness 5–6, no obvious cleavage, fine-grained crystalline texture, and specific gravity about 2.7–3.0. The pale brown areas appear to be weathered rind or adhered sediment rather than a distinct mineral.
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Physical properties
Dark gray-green, dull to slightly waxy surface with rounded weathering; likely hardness 5–6, no obvious cleavage, fine-grained crystalline texture, and specific gravity about 2.7–3.0. The pale brown areas appear to be weathered rind or adhered sediment rather than a distinct mineral.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, then became rounded by transport and weathering. Its precise age and identity cannot be determined from the photograph alone.
Uses & applications
Commonly used as aggregate, riprap, road stone, landscaping material, and occasionally carving stone; this ordinary cobble has little commercial or jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust, while andesite is common in volcanic arcs. Weathering can produce brown iron-oxide coatings and soften the outer surface.
Field identification & locations
Test a fresh edge for a fine-grained texture, streakless dark gray interior, and lack of visible quartz; basalt is usually slightly magnetic, whereas andesite may be weakly magnetic. Common worldwide in volcanic regions and stream gravels.
