igneous
Likely weathered basalt pebble
Basalt, a fine-grained mafic volcanic rock; commonly composed of plagioclase, pyroxene, and minor olivine
AI-generated identification · may be incorrectDark gray-brown, dull to weakly granular, and apparently fine-grained with a rough weathered surface; typical hardness is about 6 Mohs, with no visible cleavage and generally massive texture. The exact mineralogy cannot be confirmed from the photograph alone.
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Physical properties
Dark gray-brown, dull to weakly granular, and apparently fine-grained with a rough weathered surface; typical hardness is about 6 Mohs, with no visible cleavage and generally massive texture. The exact mineralogy cannot be confirmed from the photograph alone.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood basalts. Most examples range from recent to Precambrian in age; this specimen is likely a naturally weathered stream or beach pebble.
Uses & applications
Used extensively as crushed aggregate, road base, railroad ballast, riprap, and building stone; attractive polished basalt is occasionally used decoratively. Small ordinary pieces have minimal commercial value.
Geological facts
Its dark color and fine texture reflect rapid cooling and mafic composition. Weathering commonly produces brown, rusty, or greenish coatings from oxidation of iron-bearing minerals.
Field identification & locations
In the field, test for a dense, dark, fine-grained rock with no obvious layers or large crystals; a streak, hardness, and fresh broken surface help distinguish it from slag or sedimentary stone. Common worldwide, especially volcanic regions and river or beach gravels.
