igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with minor olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to brown-black, dense, fine-grained, and subvitreous to dull; Mohs hardness about 6, no visible cleavage, crystalline interlocking texture, and specific gravity roughly 2.8–3.0. The smooth shape and sheen indicate prolonged water abrasion or polishing.
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Physical properties
Typically dark gray to brown-black, dense, fine-grained, and subvitreous to dull; Mohs hardness about 6, no visible cleavage, crystalline interlocking texture, and specific gravity roughly 2.8–3.0. The smooth shape and sheen indicate prolonged water abrasion or polishing.
Formation & geological history
Forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface; individual basalt flows range from Precambrian to modern, so this specimen’s age cannot be determined visually. Its rounded form likely developed in a river, beach, or glacial environment.
Uses & applications
Common basalt is used as crushed aggregate, road material, building stone, railway ballast, and occasionally as a polished decorative stone. This small pebble has modest specimen value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms from mid-ocean-ridge or hotspot volcanism. Brown surfaces may reflect weathering and iron oxidation.
Field identification & locations
Identify it by its dark, heavy, fine-grained appearance, lack of large crystals, and resistance to scratching with a steel knife; a streak test is usually gray-black. Common worldwide, especially volcanic regions, rivers, beaches, and glacial deposits.
