igneous
Rounded basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to faintly subvitreous, with no visible crystals; hardness about 6 Mohs, massive texture, poor or absent cleavage, and specific gravity around 2.8–3.0.
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Physical properties
Dark gray, fine-grained, dull to faintly subvitreous, with no visible crystals; hardness about 6 Mohs, massive texture, poor or absent cleavage, and specific gravity around 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity; the smooth shape reflects stream or beach abrasion.
Uses & applications
Used as crushed aggregate, road base, railway ballast, riprap, and dimension stone; rounded specimens are inexpensive geological or landscaping collectibles.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, volcanic islands, and ocean-floor seafloor spreading zones.
Field identification & locations
Identify it by its uniform dark color, fine texture, high density, and lack of layering; a streak test is usually gray, and it should not fizz with dilute hydrochloric acid. Common worldwide near volcanic terrain and river gravels.
