igneous
Basalt pebble
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectDark gray to black, fine-grained, dull to weakly lustered, and apparently rounded/waterworn; hardness about 5–6 Mohs, with no visible cleavage or crystals. Its dense, compact texture is consistent with basalt, though the photograph alone cannot confirm composition.
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Physical properties
Dark gray to black, fine-grained, dull to weakly lustered, and apparently rounded/waterworn; hardness about 5–6 Mohs, with no visible cleavage or crystals. Its dense, compact texture is consistent with basalt, though the photograph alone cannot confirm composition.
Formation & geological history
An extrusive igneous rock formed when mafic lava rapidly cooled at or near Earth’s surface, commonly during volcanic eruptions; its exact geological age is indeterminate. The smooth shape likely results from river, beach, or glacial abrasion.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, riprap, and occasionally building stone; attractive polished pieces are sold as landscaping or lapidary material. This ordinary pebble has minimal collector value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the oceanic crust. It is typically denser and darker than andesite or granite.
Field identification & locations
Identify it by its black-gray color, fine-grained massive texture, high density, and lack of visible quartz; a streak, magnetism test, and fresh broken surface can help distinguish it from slag or weathered stone. Common worldwide, especially volcanic islands, ocean floors, and flood-basalt regions.
