igneous
Tumbled basalt (probable)
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine and iron oxides
AI-generated identification · may be incorrectDark gray to black, submetallic to dull luster, rounded and polished; typically Mohs 5–6, dense (specific gravity ~2.8–3.1), with very fine crystals and no visible cleavage. The lighter brown-gray patches may be weathered mineral grains or vesicle fillings.
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Physical properties
Dark gray to black, submetallic to dull luster, rounded and polished; typically Mohs 5–6, dense (specific gravity ~2.8–3.1), with very fine crystals and no visible cleavage. The lighter brown-gray patches may be weathered mineral grains or vesicle fillings.
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands; specimens may range from recent to Precambrian in age.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, building stone, and decorative tumbled material; generally low jewelry value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the ocean floor. This rounded specimen is likely water-worn or commercially tumbled.
Field identification & locations
Identify by its heavy, black fine-grained appearance, lack of obvious large crystals, and inability to scratch glass (usually); a streak, magnetism, and close inspection could distinguish it from slag, basaltic breccia, or iron-rich meteorite. Common worldwide, especially volcanic regions and beaches.
