igneous
Black basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectDark gray-black, fine-grained, massive, and weakly to moderately lustrous when wet or polished; Mohs hardness about 5–6, with no obvious cleavage and typical density of 2.8–3.0 g/cm³. The rusty yellow-brown patch is likely weathered iron-bearing minerals or an alteration coating.
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Physical properties
Dark gray-black, fine-grained, massive, and weakly to moderately lustrous when wet or polished; Mohs hardness about 5–6, with no obvious cleavage and typical density of 2.8–3.0 g/cm³. The rusty yellow-brown patch is likely weathered iron-bearing minerals or an alteration coating.
Formation & geological history
Forms when low-silica basaltic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; specimens may range from recent to Precambrian in age. The rounded shape indicates stream, beach, or glacial transport.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, concrete aggregate, and dimension stone; attractive pieces are used decoratively. This small common pebble has minimal commercial or collector value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. It is commonly magnetic if it contains magnetite.
Field identification & locations
Identify it by its uniformly dark, fine texture, high density, and absence of visible large crystals; a streak test is usually gray-black, and a magnet may show weak attraction. Common worldwide, especially near volcanic regions, rivers, and beaches.
