igneous
Basalt cobble
Basalt; predominantly plagioclase, clinopyroxene, and often olivine or iron-titanium oxides
AI-generated identification · may be incorrectDark gray, fine-grained, dull to weakly subvitreous luster, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape indicates water or glacial abrasion; surface may be weathered.
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Physical properties
Dark gray, fine-grained, dull to weakly subvitreous luster, with no visible crystals or cleavage; hardness about 5–6 Mohs and specific gravity roughly 2.8–3.0. The rounded shape indicates water or glacial abrasion; surface may be weathered.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Exact age and source cannot be determined from the photograph.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally building or landscaping stone; ordinary cobbles have negligible jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms lava flows, dikes, and pillow lavas. This specimen may also be a dark andesite or fine-grained metamorphic rock, but its appearance best fits basalt.
Field identification & locations
Common in volcanic regions, river gravels, beaches, and glacial deposits. Identify by its uniformly dark fine grain, heft, and lack of obvious layering; a streak, magnet test, and hand lens can improve confidence.
