igneous
Vesicular basalt pebble
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and often olivine
AI-generated identification · may be incorrectDark brown-gray, fine-grained, dull to slightly earthy luster, with numerous small vesicles (gas cavities); hardness about 5–6 Mohs and no obvious cleavage. Its rounded shape suggests natural abrasion or tumbling, and the surface appears weathered or iron-stained.
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Physical properties
Dark brown-gray, fine-grained, dull to slightly earthy luster, with numerous small vesicles (gas cavities); hardness about 5–6 Mohs and no obvious cleavage. Its rounded shape suggests natural abrasion or tumbling, and the surface appears weathered or iron-stained.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping volcanic gas bubbles; likely Cenozoic or older, though age cannot be determined visually. Common in volcanic terrains, lava flows, and stream or beach gravels.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally as a decorative or lapidary material; this small porous specimen has little jewelry value.
Geological facts
Vesicles may later fill with minerals such as zeolite, calcite, or quartz, producing amygdaloidal basalt. The brown coloration commonly results from oxidation of iron-bearing minerals.
Field identification & locations
Identify by its fine texture, dark mafic color, lightweight porous structure, and lack of visible large crystals; a magnet may show weak attraction if magnetite is present. Common worldwide in volcanic regions and transported gravels.
