igneous
Rounded volcanic pebble, likely basalt
Basalt; fine-grained mafic rock composed mainly of plagioclase and pyroxene, commonly with olivine
AI-generated identification · may be incorrectGray-green, dull to slightly earthy luster, fine-grained and vesicular with small pores; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.0.
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Physical properties
Gray-green, dull to slightly earthy luster, fine-grained and vesicular with small pores; hardness about 5–6 Mohs, no visible cleavage, and typical specific gravity around 2.8–3.0.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, likely within the Cenozoic or older volcanic history, then rounded by stream or wave transport. Exact age and provenance cannot be determined from the photograph.
Uses & applications
Commonly used as aggregate, roadstone, riprap, and landscaping material; attractive polished pieces may be sold as inexpensive décor or curiosities.
Geological facts
The rounded shape indicates abrasion during transport, while the tiny holes are gas vesicles preserved in the lava. Green coloration may reflect olivine, chlorite, or weathering.
Field identification & locations
Look for a heavy, dark, fine-grained pebble with scattered vesicles and no obvious mineral grains; it commonly occurs near volcanic terrains, beaches, and river gravel beds. A simple streak, magnet, and acid test can help distinguish it from slag or other dark rocks.
