igneous
Weathered basalt or mafic volcanic rock
Basalt; predominantly plagioclase feldspar, pyroxene, and commonly olivine or iron oxides
AI-generated identification · may be incorrectFine-grained, dark gray-brown and strongly weathered, with dull to weakly earthy luster and irregular fracture; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.2. The rounded shape is from abrasion, likely in water or sediment.
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Physical properties
Fine-grained, dark gray-brown and strongly weathered, with dull to weakly earthy luster and irregular fracture; estimated Mohs hardness 5–6 and specific gravity about 2.8–3.2. The rounded shape is from abrasion, likely in water or sediment.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly from volcanic eruptions or oceanic crust formation. Its exact age cannot be determined from the image; surface weathering has altered the exterior.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and occasionally building stone. This specimen has little jewelry value but may be useful as a representative volcanic or beach-worn sample.
Geological facts
The rusty brown coating likely reflects oxidation of iron-bearing minerals. Some basalt contains vesicles, but the visible pits here may mainly be weathering cavities.
Field identification & locations
Common in volcanic regions, oceanic islands, river gravels, and beaches; notable sources include Hawaii, Iceland, India, and the Columbia River region. Confirm with a fresh broken surface, magnetism, and a hand lens: basalt should be compact, fine-grained, and often weakly magnetic.
