igneous
Rounded basalt cobble
Basalt; predominantly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly vitreous, with Mohs hardness about 6; typically massive with no visible cleavage, though vesicles or weathering pits may occur. The smooth elongated shape appears water-worn rather than naturally crystalline.
Identified More igneous →
Explore Rounded basalt cobble in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray to black, fine-grained, dull to slightly vitreous, with Mohs hardness about 6; typically massive with no visible cleavage, though vesicles or weathering pits may occur. The smooth elongated shape appears water-worn rather than naturally crystalline.
Formation & geological history
Basalt forms when low-silica mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. The specimen’s exact age and source cannot be determined from the image; its rounded form likely reflects stream, beach, or glacial transport.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and building stone; attractive polished pieces are sold as landscaping or decorative specimens. This ordinary cobble generally has little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. A hand lens may reveal tiny pale plagioclase crystals, dark pyroxene, or small gas cavities.
Field identification & locations
Identify it by its heavy, dark, fine-grained texture, lack of layering, and resistance to scratching with a steel knife; it should not fizz with dilute hydrochloric acid. Common worldwide near volcanic regions and as transported river or beach cobbles.
