igneous
Likely basaltic river cobble
Fine- to medium-grained mafic igneous rock, commonly basalt (plagioclase–pyroxene ± olivine)
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to weakly subvitreous, with indistinct crystals and possible vesicles; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.1. Its rounded shape indicates water-worn transport; exact identification is limited by the photograph.
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Physical properties
Dark gray to brown, fine-grained, dull to weakly subvitreous, with indistinct crystals and possible vesicles; hardness about 5–6 Mohs, no obvious cleavage, specific gravity roughly 2.8–3.1. Its rounded shape indicates water-worn transport; exact identification is limited by the photograph.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly from volcanic eruptions or seafloor spreading. The cobble was later weathered and rounded in a stream, beach, or glacial environment; its geological age cannot be determined visually.
Uses & applications
Used as aggregate, road base, riprap, railroad ballast, and occasionally dimension stone. Ordinary cobbles have minimal jewelry or collector value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Vesicles, if present, represent gas bubbles trapped during cooling.
Field identification & locations
Look for a dense dark matrix, tiny pale plagioclase crystals, and occasional holes; a streak, fresh break, and magnetic response can help distinguish it from andesite or dark metamorphic rocks. Common worldwide in volcanic terrains and river gravels.
