igneous
Basalt cobble
Fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to black, massive, and fine-grained with a dull to slight waxy luster; Mohs hardness about 5–6, with no visible cleavage and irregular fracture. The smoothly rounded edges indicate water or glacial abrasion, and the surface may be weathered or artificially polished.
Identified More igneous →
Explore Basalt cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically dark gray to black, massive, and fine-grained with a dull to slight waxy luster; Mohs hardness about 5–6, with no visible cleavage and irregular fracture. The smoothly rounded edges indicate water or glacial abrasion, and the surface may be weathered or artificially polished.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. It occurs in lava flows, oceanic crust, volcanic islands, and glacial or river gravels.
Uses & applications
Used extensively as crushed aggregate, road base, riprap, railway ballast, and dimension stone; attractive dense pieces may be used decoratively. This ordinary cobble has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Identify it by its uniform dark color, fine texture, high density, and lack of obvious individual crystals; a streak test is usually pale gray, and it commonly does not fizz with dilute acid. Common in volcanic regions and transported gravel deposits.
