igneous
Dark river pebble, likely basalt
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectHardness about 6 on the Mohs scale; dark gray to black, fine-grained, dull to subtly glassy luster, compact texture, and no visible cleavage. The rounded, smooth shape indicates water abrasion and possible polishing.
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Physical properties
Hardness about 6 on the Mohs scale; dark gray to black, fine-grained, dull to subtly glassy luster, compact texture, and no visible cleavage. The rounded, smooth shape indicates water abrasion and possible polishing.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, generally in Cenozoic or older volcanic terrains. This specimen was likely transported and rounded in a river or beach environment.
Uses & applications
Used as crushed aggregate, road material, railroad ballast, paving stone, and occasionally decorative landscaping stone; this small pebble has minimal jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Its dark color reflects abundant ferromagnesian minerals and relatively low silica content.
Field identification & locations
Identify it by its uniform dark color, fine texture, high density, and lack of obvious layering or large crystals; a streak test is usually gray. Common in volcanic regions, river gravels, and beaches worldwide.
