igneous
Rounded basalt cobble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectDark gray, fine-grained, massive, and dull to subtly vitreous; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The rounded shape and surface abrasion indicate stream or beach transport.
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Physical properties
Dark gray, fine-grained, massive, and dull to subtly vitreous; typically Mohs 5–6, with no visible cleavage and specific gravity about 2.8–3.0. The rounded shape and surface abrasion indicate stream or beach transport.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface, generally in volcanic regions and oceanic crust. Its exact geological age cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road material, riprap, railway ballast, and occasionally dimension stone; attractive dense pieces are collected but rarely used as gemstones.
Geological facts
Basalt commonly contains microscopic crystals, so it may appear nearly uniform even though it is crystalline. Small pale flecks may be feldspar, while dark metallic grains can be magnetite.
Field identification & locations
Identify it by its dark color, high density, fine texture, and resistance to scratching; it should not fizz with dilute hydrochloric acid. Common sources include volcanic terrains, river gravels, and glacial deposits.
