igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock, mainly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to black, brown/rust-weathered surface, dull to weakly subvitreous luster, very fine-grained interlocking crystals, and no obvious cleavage. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Identified More igneous →
Explore Weathered basalt cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically Mohs 5–6, dark gray to black, brown/rust-weathered surface, dull to weakly subvitreous luster, very fine-grained interlocking crystals, and no obvious cleavage. The rounded shape and smooth surface indicate natural water abrasion or tumbling.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic events; the specimen’s exact age cannot be determined visually. Rust-colored areas reflect oxidation of iron-bearing minerals during weathering.
Uses & applications
Used widely as crushed aggregate, road stone, riprap, ballast, and occasionally landscaping stone; attractive pieces may be collected or polished, but this example has little gem value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms pillow lavas, lava flows, and volcanic plateaus.
Field identification & locations
Identify it by its high density, dark fine-grained body, lack of visible quartz, and resistance to scratching; a steel knife may scratch weathered surfaces but usually not fresh basalt. Common worldwide, especially in volcanic regions and stream gravels.
