igneous
Water-worn dark pebble, likely basalt
Basalt (fine-grained mafic volcanic rock; identification is provisional from the image)
AI-generated identification · may be incorrectSmooth, rounded, dark gray pebble with a subtle fine-grained surface and a small pale vein or mineral patch. Basalt is typically Mohs 5–6, dull to slightly silky, and composed mainly of plagioclase and pyroxene, often with olivine; individual crystals are usually too small to see.
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Physical properties
Smooth, rounded, dark gray pebble with a subtle fine-grained surface and a small pale vein or mineral patch. Basalt is typically Mohs 5–6, dull to slightly silky, and composed mainly of plagioclase and pyroxene, often with olivine; individual crystals are usually too small to see.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface; basalt ranges from recent to billions of years old. The pebble’s rounded shape suggests abrasion in a stream or on a shore.
Uses & applications
Commonly used as crushed aggregate, roadstone, and construction stone; attractive smooth pebbles are also used decoratively or collected.
Geological facts
The pale streak may be a mineral-filled fracture, such as quartz or calcite, but cannot be identified reliably from the photo.
Field identification & locations
Look for a dense, dark, fine-grained texture and no obvious large crystals; basalt commonly occurs in volcanic regions and river or beach gravels. A fresh broken surface and simple hardness or streak tests would help confirm it.
