igneous
Dark weathered pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; typically plagioclase, pyroxene, and olivine)
AI-generated identification · may be incorrectDark gray to black, dull to weakly subvitreous, fine-grained and massive; Mohs hardness about 5–6, with no obvious cleavage. The rounded shape indicates water or abrasion weathering; exact identification is limited by the image.
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Physical properties
Dark gray to black, dull to weakly subvitreous, fine-grained and massive; Mohs hardness about 5–6, with no obvious cleavage. The rounded shape indicates water or abrasion weathering; exact identification is limited by the image.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood basalts. Most examples range from relatively young volcanic rocks to Precambrian material, depending on locality.
Uses & applications
Used extensively as crushed aggregate, road base, railway ballast, riprap, and building stone; attractive polished basalt is used decoratively but this pebble has little commercial value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. It is commonly denser and darker than andesite or granite.
Field identification & locations
Look for a uniformly dark, fine-grained texture, high density, and weak attraction to a magnet if iron-bearing; a fresh surface is more diagnostic than the weathered exterior. Common worldwide, especially near volcanic regions and stream or beach gravels.
