igneous
Weathered basalt or basaltic cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, and dull to weakly glassy, with tan-orange iron-oxide weathering stains; hardness about 6, no obvious cleavage, and typically specific gravity 2.8–3.0. The rounded shape indicates transport and abrasion, while the pale patches may be weathered minerals or adhering sediment.
Identified More igneous →
Explore Weathered basalt or basaltic cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray, fine-grained, and dull to weakly glassy, with tan-orange iron-oxide weathering stains; hardness about 6, no obvious cleavage, and typically specific gravity 2.8–3.0. The rounded shape indicates transport and abrasion, while the pale patches may be weathered minerals or adhering sediment.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph; the surface weathering is likely much younger than the rock.
Uses & applications
Commonly used as crushed aggregate, road base, railway ballast, riprap, and occasionally dimension stone. This ordinary cobble has little jewelry or commercial collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Iron-bearing minerals commonly weather to yellow, brown, or red oxides.
Field identification & locations
Identify it by its heavy, dark, fine-grained texture and lack of visible large crystals; a streak, magnetism test, and fresh broken surface would help distinguish basalt from slag or other dark rocks. Common in volcanic regions and stream or glacial gravels.
