igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark olive-gray, dull to weakly earthy luster, very fine-grained and apparently massive; hardness about 5–6 Mohs, typically no visible cleavage, and specific gravity roughly 2.8–3.0. Its rounded triangular shape and smooth surface indicate natural abrasion or tumbling.
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Physical properties
Dark olive-gray, dull to weakly earthy luster, very fine-grained and apparently massive; hardness about 5–6 Mohs, typically no visible cleavage, and specific gravity roughly 2.8–3.0. Its rounded triangular shape and smooth surface indicate natural abrasion or tumbling.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic islands, and flood-basalt provinces. The specimen’s exact age and source cannot be determined from the photograph.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and occasionally as paving or landscaping stone; small pieces are also sold as curios or craft material.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up most oceanic crust. Its dark color reflects abundant ferromagnesian minerals rather than carbon content.
Field identification & locations
Look for a dense, dark, fine-grained rock that does not fizz with dilute hydrochloric acid and may attract a weak magnet if magnetite-rich; distinguishing it from dark andesite or slag requires texture and context. Common worldwide, especially volcanic regions and stream or beach gravels.
