igneous
Polished basalt pebble
Basalt, a fine-grained mafic volcanic rock; chiefly plagioclase, pyroxene, and iron-titanium oxides
AI-generated identification · may be incorrectDark black to brown, compact, fine-grained, and weakly metallic or glassy-lustrous when polished; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity 2.8–3.0. Rust-brown areas likely represent weathered iron-bearing minerals.
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Physical properties
Dark black to brown, compact, fine-grained, and weakly metallic or glassy-lustrous when polished; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity 2.8–3.0. Rust-brown areas likely represent weathered iron-bearing minerals.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. This rounded specimen was likely naturally water-worn and then polished or tumbled.
Uses & applications
Used as aggregate, road stone, building stone, landscaping material, and decorative pebble; attractive pieces are sold as polishable lapidary material.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant magnesium- and iron-rich minerals.
Field identification & locations
Identify by its dark, dense, fine-grained appearance, lack of large crystals, and strong response to a steel scratch test only if unpolished; unlike slag, it usually lacks bubbles. Common worldwide near volcanic terrains and river gravels.
