igneous
Likely basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to slightly vitreous; typically dense, with hardness about 5–6 Mohs and no visible cleavage. The rounded shape indicates water or abrasion wear; exact identification requires testing and magnification.
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Physical properties
Dark gray to black, fine-grained, dull to slightly vitreous; typically dense, with hardness about 5–6 Mohs and no visible cleavage. The rounded shape indicates water or abrasion wear; exact identification requires testing and magnification.
Formation & geological history
Forms when mafic lava cools rapidly at or near Earth’s surface, commonly from volcanic eruptions, oceanic crust, and ancient lava flows. Basalt ranges from Precambrian to modern age and is widespread on Earth and other rocky planets.
Uses & applications
Used as aggregate, road material, riprap, construction stone, and occasionally landscaping material; small attractive pieces are inexpensive collectibles.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. Its dark color reflects abundant iron- and magnesium-rich minerals.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and crushed-stone deposits; magnetic response may occur if magnetite is present. Vesicles, glassy surfaces, or visible olivine crystals would help distinguish variants.
