igneous
Rounded gray river pebble, likely basaltic/volcanic rock
Probable basalt or andesite; exact identification requires a fresh surface and tests
AI-generated identification · may be incorrectDark gray, fine-grained, dull to slightly waxy luster, rounded by abrasion, with possible small vesicles or weathered mineral grains. Typical hardness is about 5–6 Mohs; no obvious cleavage; specific gravity roughly 2.7–3.0.
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Physical properties
Dark gray, fine-grained, dull to slightly waxy luster, rounded by abrasion, with possible small vesicles or weathered mineral grains. Typical hardness is about 5–6 Mohs; no obvious cleavage; specific gravity roughly 2.7–3.0.
Formation & geological history
Likely formed when mafic to intermediate lava cooled rapidly at or near Earth’s surface, then became rounded in a stream or shoreline. Basaltic rocks range from Precambrian to modern, so age cannot be determined visually.
Uses & applications
Commonly used as aggregate, road stone, landscaping rock, riprap, and occasionally for small carvings or polished specimens; this ordinary pebble has little commercial value.
Geological facts
Its smooth shape records prolonged transport and repeated impact in moving water or surf. A magnet test may show weak attraction if the rock contains magnetite.
Field identification & locations
Look for a uniformly fine texture, dark gray color, high density, and possible tiny holes; basalt is usually darker and denser than andesite. Common in volcanic regions and gravel beds worldwide; a streak, magnet, hardness, and thin-section test would improve identification.
