igneous
Polished basalt pebble (likely vesicular basalt)
Basalt; fine-grained mafic volcanic rock, principally plagioclase and pyroxene with possible olivine and iron oxides
AI-generated identification · may be incorrectDark brown to black, smooth polished surface, dull to weakly vitreous luster, and apparent small vesicles or pits; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity 2.8–3.0.
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Physical properties
Dark brown to black, smooth polished surface, dull to weakly vitreous luster, and apparent small vesicles or pits; hardness about 5–6 Mohs, no obvious cleavage, and typical specific gravity 2.8–3.0.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, probably within the Cenozoic or younger; rounded shape indicates water or abrasion, followed by human polishing. The dark cavities may contain weathering products or trapped sediment.
Uses & applications
Used as a decorative pebble, lapidary object, aquarium or landscaping stone, and occasionally as aggregate or building stone; this specimen has modest collectible value.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms lava flows, pillow lavas, and volcanic islands.
Field identification & locations
Identify it by its fine texture, high density, dark mafic color, and lack of visible large crystals; vesicles distinguish it from many dense river pebbles. Common sources include volcanic regions, ocean beaches, and basaltic river gravels.
