igneous
Rounded dark pebble, likely basalt or andesite
Fine-grained mafic volcanic rock; exact identification is uncertain from the image
AI-generated identification · may be incorrectDark gray to brown, dull to slightly glossy when wet, with a fine-grained matrix and possible small mineral grains. Likely hardness around 5–6 Mohs; crystal structure and cleavage are not distinguishable in the photo.
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Physical properties
Dark gray to brown, dull to slightly glossy when wet, with a fine-grained matrix and possible small mineral grains. Likely hardness around 5–6 Mohs; crystal structure and cleavage are not distinguishable in the photo.
Formation & geological history
Likely formed when lava cooled at or near Earth’s surface, then became rounded by transport in water. Its geological age and precise formation environment cannot be determined from appearance alone.
Uses & applications
Ordinary pebbles are used for landscaping, drainage, and decorative aggregate; this specimen has little commercial value unless distinctive features are confirmed.
Geological facts
Wet surfaces can make fine-grained volcanic rocks appear unusually reflective. The image does not show diagnostic features sufficient to distinguish basalt from other dark volcanic rocks.
Field identification & locations
Look for a dense, fine-grained texture and small crystals; a fresh broken surface and simple hardness or streak tests would help identify it. Dark volcanic pebbles occur in river, beach, and glacial gravels worldwide.
