igneous
Rounded basalt pebble
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, weathered, dull to faintly granular, with no visible cleavage; hardness about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or wave abrasion, and the pale speckling may be weathering or small mineral grains.
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Physical properties
Typically dark gray to brown, weathered, dull to faintly granular, with no visible cleavage; hardness about 6 on the Mohs scale and specific gravity roughly 2.8–3.0. Its smooth shape indicates water or wave abrasion, and the pale speckling may be weathering or small mineral grains.
Formation & geological history
Basalt forms when low-silica lava cools rapidly at or near Earth’s surface, commonly from oceanic volcanism, hot spots, and continental flood basalts. The pebble’s exact age cannot be determined visually; its rounded form reflects later transport in a stream, beach, or gravel deposit.
Uses & applications
Used as crushed aggregate, road base, riprap, railway ballast, and occasionally dimension stone; attractive polished pieces are collected but are not generally jewelry-grade.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the oceanic crust. It commonly attracts a magnet weakly because of iron-bearing minerals.
Field identification & locations
Identify it by its heavy, fine-grained texture, dark color, lack of obvious layers, and resistance to scratching; it should not fizz with dilute acid. Common worldwide near volcanic regions and transported river or beach gravels.
