igneous
Dark basalt cobble
Basalt, a fine-grained mafic volcanic rock
AI-generated identification · may be incorrectAppears dark gray to black, rounded and smooth, with a dull to slightly glassy luster and no obvious crystals. Typical basalt is mainly plagioclase and pyroxene, often with olivine; hardness is roughly 6 on the Mohs scale and specific gravity about 2.8–3.0.
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Physical properties
Appears dark gray to black, rounded and smooth, with a dull to slightly glassy luster and no obvious crystals. Typical basalt is mainly plagioclase and pyroxene, often with olivine; hardness is roughly 6 on the Mohs scale and specific gravity about 2.8–3.0.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface; its age cannot be determined from the image. Rounded shape likely reflects water or prolonged abrasion.
Uses & applications
Commonly used as crushed aggregate, road stone, paving, and landscaping material; attractive cobbles may be collected or used decoratively.
Geological facts
Basalt is one of Earth’s most abundant volcanic rocks and forms much of the ocean floor.
Field identification & locations
Look for a dense, dark, fine-grained rock, often with tiny crystals or vesicles; common in volcanic regions and river or beach gravels. The image alone cannot confirm basalt versus another dark rock such as andesite.
