igneous
Polished porphyritic basalt cobble
Basalt, commonly composed of plagioclase feldspar, clinopyroxene, olivine, and opaque iron-titanium oxides
AI-generated identification · may be incorrectFine-grained, dark brown to black volcanic rock with scattered lighter mineral grains and a rounded, polished surface; hardness about 5.5–6.5 Mohs, dull to subvitreous luster, no obvious cleavage, and specific gravity roughly 2.8–3.1.
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Physical properties
Fine-grained, dark brown to black volcanic rock with scattered lighter mineral grains and a rounded, polished surface; hardness about 5.5–6.5 Mohs, dull to subvitreous luster, no obvious cleavage, and specific gravity roughly 2.8–3.1.
Formation & geological history
Formed when mafic lava cooled rapidly at or near Earth’s surface, generally in volcanic flows; its exact geological age cannot be determined from the image and may range from ancient to recent.
Uses & applications
Used as crushed aggregate, road stone, riprap, and occasionally as a decorative or craft-polished stone; usually has little jewelry value.
Geological facts
The rounded elongated shape is likely water-worn and subsequently polished or naturally smoothed. Dark patches may represent glassy matrix, weathering, or concentrations of mafic minerals.
Field identification & locations
Identify by its high density, dark fine-grained matrix, lack of visible layering, and attraction to a strong magnet if magnetite is abundant; common in volcanic regions, river gravels, and glacial deposits.
