igneous
Basalt cobble
Basalt; fine-grained mafic volcanic rock composed mainly of plagioclase feldspar and pyroxene, commonly with olivine and iron-titanium oxides
AI-generated identification · may be incorrectDark brown to gray, weathered and rounded, with dull to faintly vitreous luster and abundant tiny vesicles or weathering pits. Mohs hardness about 5–6; massive, fine-grained texture with no visible cleavage and specific gravity around 2.8–3.0.
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Physical properties
Dark brown to gray, weathered and rounded, with dull to faintly vitreous luster and abundant tiny vesicles or weathering pits. Mohs hardness about 5–6; massive, fine-grained texture with no visible cleavage and specific gravity around 2.8–3.0.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, likely millions to billions of years ago depending on its source. The rounded shape indicates prolonged transport and abrasion in a river, beach, or glacial environment.
Uses & applications
Used as crushed aggregate, roadstone, railway ballast, riprap, and occasionally building or landscaping stone. This weathered cobble has modest decorative or teaching value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Vesicles record gas bubbles trapped during lava cooling.
Field identification & locations
Identify it by its heavy, dark, fine-grained appearance, magnetic response in many specimens, and lack of visible large crystals; a streak test is usually gray. Common worldwide, especially volcanic regions, river gravels, and beaches.
