igneous
Weathered basalt cobble
Basalt, a mafic volcanic rock; commonly composed of plagioclase, pyroxene, and iron-titanium oxides
AI-generated identification · may be incorrectDark brown-black, fine-grained, rounded, and weakly vesicular or weathered; typically Mohs 5–6, dull to faintly glassy luster, with no visible cleavage and interlocking microscopic crystals. The orange-brown areas are likely iron-oxide weathering.
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Physical properties
Dark brown-black, fine-grained, rounded, and weakly vesicular or weathered; typically Mohs 5–6, dull to faintly glassy luster, with no visible cleavage and interlocking microscopic crystals. The orange-brown areas are likely iron-oxide weathering.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, generally in volcanic settings from Precambrian to modern age. Its rounded shape and smooth surface result from prolonged wave or stream abrasion.
Uses & applications
Used as aggregate, road material, riprap, railway ballast, and landscaping stone; attractive polished pieces may be used decoratively but it is not typically a gemstone.
Geological facts
Basalt is the most abundant volcanic rock in oceanic crust and commonly forms pillows beneath seawater. Weathering can produce reddish limonite or hematite coatings.
Field identification & locations
Identify by its dense dark color, fine texture, lack of obvious quartz, and magnetic response if iron-rich; a streak, thin-section, or hardness test would confirm it. Common on volcanic coasts, riverbeds, and glacial deposits.
