igneous
Rounded basalt or basaltic cobble
Fine- to medium-grained mafic volcanic rock; chiefly plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray-brown, dense, and very fine- to medium-grained with a dull to slightly granular luster; typically massive, with no visible cleavage. Mohs hardness is about 5–6 and specific gravity about 2.8–3.1.
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Physical properties
Dark gray-brown, dense, and very fine- to medium-grained with a dull to slightly granular luster; typically massive, with no visible cleavage. Mohs hardness is about 5–6 and specific gravity about 2.8–3.1.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, likely during the Cenozoic or younger depending on locality. Its smooth, rounded shape indicates transport and abrasion by a stream, beach, or glacial deposit.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally dimension stone. Attractive polished pieces may be sold as landscape or teaching specimens, but this example has modest value.
Geological facts
Basalt is the most abundant volcanic rock on oceanic crust and commonly forms oceanic islands and large lava flows. The specimen may also be a related rock such as fine-grained gabbro, which cannot be distinguished confidently from the image alone.
Field identification & locations
Identify it by its dark color, high density, lack of obvious layering, and resistance to scratching; a steel file usually scratches it weakly or not at all. Common sources include volcanic regions, river gravels, beaches, and glacial tills.
