igneous
Rounded basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with minor olivine and iron oxides
AI-generated identification · may be incorrectDark gray-green, dull to subtly granular, very fine-grained and rounded by water abrasion; likely hardness about 6 on Mohs scale, with no visible cleavage or crystal faces. The small dark pits may be vesicles or weathered mafic minerals; specific gravity is typically 2.8–3.0.
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Physical properties
Dark gray-green, dull to subtly granular, very fine-grained and rounded by water abrasion; likely hardness about 6 on Mohs scale, with no visible cleavage or crystal faces. The small dark pits may be vesicles or weathered mafic minerals; specific gravity is typically 2.8–3.0.
Formation & geological history
Formed when low-silica lava rapidly cooled at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Its smooth shape indicates transport in a river, beach, or glacial environment.
Uses & applications
Used as aggregate, roadstone, riprap, railway ballast, landscaping stone, and occasionally as a polished decorative or hand specimen. This ordinary cobble has little commercial value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas. Its greenish cast may reflect chlorite or other low-grade alteration minerals.
Field identification & locations
Identify it by its dark color, fine texture, heft, lack of layering, and possible weak attraction to a magnet from iron-bearing minerals; a fresh broken surface should remain dark and fine-grained. Common in volcanic terrains and transported river gravels worldwide.
