igneous
Black basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron-titanium oxides
AI-generated identification · may be incorrectTypically Mohs 6, dark gray to black, fine-grained, and with a dull to weakly vitreous luster; the rounded, glossy surface indicates water tumbling or polishing. Cleavage is absent as a rock, and density is commonly about 2.8–3.0 g/cm³.
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Physical properties
Typically Mohs 6, dark gray to black, fine-grained, and with a dull to weakly vitreous luster; the rounded, glossy surface indicates water tumbling or polishing. Cleavage is absent as a rock, and density is commonly about 2.8–3.0 g/cm³.
Formation & geological history
Forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly from Cenozoic to modern volcanic activity, though basalt occurs throughout geological time. This specimen was likely rounded in a stream, beach, or gravel deposit.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, riprap, and decorative landscaping stone; attractive polished pieces are inexpensive collectibles.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals and opaque iron oxides.
Field identification & locations
Identify by its uniformly dark color, fine texture, substantial weight, and lack of visible large crystals; a streak test is usually gray-black, and it is not strongly magnetic unless oxide-rich. Common worldwide, especially near volcanic terrains and ocean beaches.
