igneous
Black basalt pebble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine
AI-generated identification · may be incorrectTypically dark gray to black, fine-grained, dense, and weakly to moderately magnetic; hardness about 5–6 Mohs, dull to faintly glassy luster, with no visible cleavage. The rounded, smooth surface indicates water or sediment abrasion; small pits may reflect vesicles.
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Physical properties
Typically dark gray to black, fine-grained, dense, and weakly to moderately magnetic; hardness about 5–6 Mohs, dull to faintly glassy luster, with no visible cleavage. The rounded, smooth surface indicates water or sediment abrasion; small pits may reflect vesicles.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, commonly in oceanic crust, lava flows, and volcanic islands. Most basalt is Phanerozoic or younger, though ancient examples occur worldwide; this specimen’s age cannot be determined visually.
Uses & applications
Used as crushed aggregate, road stone, riprap, railway ballast, and building stone; polished basalt is also used decoratively. Common pebbles usually have little commercial collector value.
Geological facts
Commonly contains iron- and magnesium-rich minerals, giving it its dark color and relatively high density. A magnet test, streak test, and fresh-break examination help distinguish it from black chert or slag.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits; major sources include Hawaii, Iceland, India, the Columbia River region, and oceanic islands. Confirm with a fresh surface, lack of conchoidal fracture, fine texture, and possible magnetism.
