igneous
Weathered basalt pebble
Basalt, a mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dense, and nonmetallic with dull to faintly vitreous luster; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates water or sediment abrasion; brown coloration is likely weathering or iron-oxide staining.
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Physical properties
Typically dark gray to black, fine-grained, dense, and nonmetallic with dull to faintly vitreous luster; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded, smooth surface indicates water or sediment abrasion; brown coloration is likely weathering or iron-oxide staining.
Formation & geological history
Formed when low-silica, iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. The specimen was likely transported and rounded in a river, beach, or glacial deposit.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and occasionally dimension stone; attractive polished pieces are sold as inexpensive collectibles. It has little jewelry value compared with ornamental varieties such as basalt with vesicles or zeolites.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust, including mid-ocean ridges and many ocean-island volcanoes.
Field identification & locations
Identify it by its heavy, fine-grained black appearance, lack of obvious large crystals, and inability to scratch glass easily; a streak, magnetism test, and hand lens can help distinguish it from slag or dark sedimentary rock. Common sources include volcanic regions, river gravels, beaches, and glacial deposits.
