igneous
Basalt pebble
Basalt; primarily plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly glassy, generally massive, with small vesicles and weathering pits. Hardness is about 6 on Mohs scale; cleavage is not apparent in the whole rock and specific gravity is approximately 2.8–3.0.
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Physical properties
Dark gray to black, fine-grained, dull to weakly glassy, generally massive, with small vesicles and weathering pits. Hardness is about 6 on Mohs scale; cleavage is not apparent in the whole rock and specific gravity is approximately 2.8–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during volcanic eruptions. The rounded shape indicates transport and abrasion, likely in a stream, on a beach, or by glacial action.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, paving stone, and decorative landscaping material; attractive polished pieces may be sold as low-cost specimens.
Geological facts
Its tiny holes are vesicles—gas bubbles trapped as lava solidified. Basalt makes up much of the oceanic crust and forms extensive volcanic plateaus and flows.
Field identification & locations
Identify it by its black, dense, fine-grained appearance, heavier feel, lack of visible large crystals, and possible vesicles; a streak test is usually gray to black. Common worldwide in volcanic regions, oceanic islands, and river or beach gravels.
