igneous
Tumbled basalt
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectTypically dark gray to brown-black, fine-grained, dull to slightly glassy, with hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded, glossy surface indicates human tumbling or polishing; small pits may reflect vesicles or weathering.
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Physical properties
Typically dark gray to brown-black, fine-grained, dull to slightly glassy, with hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded, glossy surface indicates human tumbling or polishing; small pits may reflect vesicles or weathering.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic plateaus, and lava flows. Individual specimens may range from geologically recent to billions of years old depending on source.
Uses & applications
Used extensively as crushed aggregate, road stone, concrete material, railroad ballast, and dimension stone; polished pieces are sold as decorative or lapidary specimens.
Geological facts
Its dark color reflects abundant mafic minerals and iron-bearing oxides. Basalt can be magnetic if it contains enough magnetite.
Field identification & locations
Identify it by its dense dark color, fine texture, lack of obvious large crystals, and high heft; a magnet may show attraction. Common worldwide, especially in volcanic regions and river or beach gravels.
