igneous
Rounded basalt pebble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectDark gray-brown, rounded and slightly weathered, with a dull to faintly granular luster. Typically Mohs 5–6, compact, fine-grained, with no visible cleavage; specific gravity about 2.8–3.0.
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Physical properties
Dark gray-brown, rounded and slightly weathered, with a dull to faintly granular luster. Typically Mohs 5–6, compact, fine-grained, with no visible cleavage; specific gravity about 2.8–3.0.
Formation & geological history
Formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic events. Its smooth shape likely resulted from abrasion in a stream, beach, or glacial environment.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally for carvings or educational specimens; this example has little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the oceanic crust, including mid-ocean ridges and many ocean-island volcanoes.
Field identification & locations
Identify it by its uniformly dark color, heavy feel, fine texture, and resistance to scratching with a steel knife; it is commonly found in volcanic terrains, river gravels, beaches, and glacial deposits. The photograph alone cannot exclude fine-grained andesite or weathered gabbro.
