igneous
Weathered basalt pebble
Basalt; predominantly plagioclase, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectDark brown-black, fine-grained, subrounded and weathered, with dull to slightly glassy luster and no visible cleavage; typical hardness about 5–6 Mohs and specific gravity 2.8–3.0.
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Physical properties
Dark brown-black, fine-grained, subrounded and weathered, with dull to slightly glassy luster and no visible cleavage; typical hardness about 5–6 Mohs and specific gravity 2.8–3.0.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly, generally during volcanic episodes ranging from recent to billions of years old; this specimen was rounded by water or abrasion.
Uses & applications
Used as crushed aggregate, road base, riprap, ballast, and decorative stone; ordinary pebbles have minimal commercial value.
Geological facts
Its brown surface likely reflects oxidation of iron-bearing minerals and weathering. The dark matrix with scattered small pale crystals is characteristic of basalt rather than granite.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits worldwide. Identify it by its heavy, dark, fine-grained texture, weak magnetism in some pieces, and lack of obvious layered structure; a hand lens or thin section is needed for certainty.
