igneous
Polished basalt pebble
Vesicular to amygdaloidal basalt, composed mainly of plagioclase, pyroxene, and iron-rich oxides
AI-generated identification · may be incorrectDark gray-black, fine-grained, rounded and polished, with pale circular vesicles or mineral-filled amygdules; typically hardness 5–6, dull to subvitreous luster, massive texture, and no obvious cleavage. The visible rounded cavities are characteristic of gas bubbles later filled or partly filled by secondary minerals.
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Physical properties
Dark gray-black, fine-grained, rounded and polished, with pale circular vesicles or mineral-filled amygdules; typically hardness 5–6, dull to subvitreous luster, massive texture, and no obvious cleavage. The visible rounded cavities are characteristic of gas bubbles later filled or partly filled by secondary minerals.
Formation & geological history
A volcanic extrusive rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic episodes. Vesicles formed from escaping gas; amygdules developed later when groundwater deposited minerals in the cavities.
Uses & applications
Used as aggregate, road stone, landscaping material, ballast, and decorative polished stone; generally unsuitable as a gemstone except as a collectible pebble or lapidary piece.
Geological facts
Basalt is the most abundant volcanic rock and forms much of the oceanic crust. The specimen’s glossy surface appears tumbled or polished, so its natural texture and fresh color may be partly altered.
Field identification & locations
Identify by its heavy feel, dark color, fine grain, magnetic response in some specimens, and rounded cavities; common in volcanic regions, river gravels, beaches, and glacial deposits. Exact identification would require a fresh surface or thin-section examination.
