igneous
Dark rounded pebble, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase, pyroxene, and often olivine
AI-generated identification · may be incorrectBlack to dark gray, dull to weakly glassy luster, fine-grained texture, and typically massive with no visible cleavage; hardness about 6, specific gravity roughly 2.8–3.0. The smooth, oval shape indicates abrasion by water or sediment movement.
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Physical properties
Black to dark gray, dull to weakly glassy luster, fine-grained texture, and typically massive with no visible cleavage; hardness about 6, specific gravity roughly 2.8–3.0. The smooth, oval shape indicates abrasion by water or sediment movement.
Formation & geological history
Basalt forms when low-viscosity mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields, oceanic crust, and flood-basalt provinces. Its exact age cannot be determined from the photograph and may range from recent to billions of years old.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are occasionally collected but are usually low-value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and makes up most oceanic crust. Some varieties contain vesicles, magnetic iron minerals, or small olivine crystals.
Field identification & locations
A likely basalt pebble is dense, opaque, non-foliated, and generally does not fizz with dilute hydrochloric acid; a streak test gives a gray-black streak. Common in volcanic regions and stream or beach gravels; laboratory testing is needed to distinguish it from dark andesite, gabbro, or slag.
