igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectHardness about 6 on the Mohs scale; dark gray-brown, dull to faintly earthy luster, fine-grained and massive, with irregular fractures and rounded weathering surfaces. The rusty-brown coloration likely reflects iron-oxide weathering.
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Physical properties
Hardness about 6 on the Mohs scale; dark gray-brown, dull to faintly earthy luster, fine-grained and massive, with irregular fractures and rounded weathering surfaces. The rusty-brown coloration likely reflects iron-oxide weathering.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the rounded shape indicates stream, beach, or glacial abrasion. Exact age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, riprap, ballast, and occasionally dimension stone or landscaping material; generally not used as a gemstone.
Geological facts
Basalt is Earth’s most common volcanic rock and forms much of the oceanic crust. Its iron- and magnesium-rich minerals weather to brown or reddish coatings and soils.
Field identification & locations
Identify by its heavy feel, fine grain, dark mafic color, lack of visible large crystals, and resistance to scratching with a steel knife; it typically does not fizz with dilute acid. Common in volcanic regions and basalt-rich river or beach gravels.
