igneous
Weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to brown, dense, fine-grained, and dull to weakly glassy; hardness about 6–7 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and surface pits indicate water abrasion and weathering; the bright flecks may be weathered feldspar or metal oxides.
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Physical properties
Typically dark gray to brown, dense, fine-grained, and dull to weakly glassy; hardness about 6–7 Mohs, no visible cleavage, and specific gravity roughly 2.8–3.1. The rounded shape and surface pits indicate water abrasion and weathering; the bright flecks may be weathered feldspar or metal oxides.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly at oceanic ridges, hotspots, and volcanic fields. The specimen’s exact geological age cannot be determined from the photograph and may range from recent to hundreds of millions of years.
Uses & applications
Used as crushed aggregate, road base, railway ballast, concrete stone, and occasionally as ornamental or landscaping rock. This ordinary pebble has little jewelry use.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Vesicles or small holes can develop from gas bubbles trapped during lava cooling.
Field identification & locations
Identify it by its dark, heavy, fine-grained texture and lack of obvious layering or large crystals; a streak test is usually gray, and it generally does not react with dilute hydrochloric acid. Common sources include volcanic regions, riverbeds, beaches, and glacial deposits.
