igneous
Dark volcanic pebble, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase, pyroxene, and commonly olivine or magnetite
AI-generated identification · may be incorrectTypically black to dark brown, fine-grained, with dull to subtly glassy luster; hardness about 5–6 Mohs, no visible cleavage, and relatively high density (about 2.8–3.0 g/cm³). The rounded, smooth shape indicates water abrasion or tumbling.
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Physical properties
Typically black to dark brown, fine-grained, with dull to subtly glassy luster; hardness about 5–6 Mohs, no visible cleavage, and relatively high density (about 2.8–3.0 g/cm³). The rounded, smooth shape indicates water abrasion or tumbling.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly from volcanic flows or oceanic crust; specimens may range from modern to billions of years old. The image alone cannot establish its exact age or locality.
Uses & applications
Commonly used as crushed aggregate, road stone, railway ballast, riprap, and occasionally building or landscaping stone; small polished pieces are inexpensive decorative specimens.
Geological facts
Its dark color and fine texture reflect rapid cooling of iron- and magnesium-rich magma. Vesicles, if present, may indicate trapped volcanic gas, but none are clearly visible here.
Field identification & locations
Identify by its heavy feel, dark fine-grained texture, lack of obvious crystals, and usually weak response to a magnet if magnetite-rich; distinguish it from industrial slag by its uniform natural texture and absence of obvious bubbles or metallic glass. Common worldwide, especially volcanic regions and stream beaches.
