igneous
Tumbled basalt with vesicles and mineral infill
Basalt, commonly composed of plagioclase, pyroxene, and iron-rich oxides; dark patches may include zeolite, quartz, or calcite
AI-generated identification · may be incorrectFine-grained, dark brown to black, rounded and polished; typically dull to faintly glassy, hardness about 5–6 Mohs, with no obvious cleavage. The surface shows possible vesicles or weathered cavities containing lighter secondary minerals.
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Physical properties
Fine-grained, dark brown to black, rounded and polished; typically dull to faintly glassy, hardness about 5–6 Mohs, with no obvious cleavage. The surface shows possible vesicles or weathered cavities containing lighter secondary minerals.
Formation & geological history
Basalt forms when low-silica lava rapidly cools at or near Earth’s surface, commonly in volcanic fields and oceanic crust. Most examples range from geologically young to several billion years old; this specimen is likely a modern tumbled fragment.
Uses & applications
Used as aggregate, road stone, building material, and decorative stone; attractive polished pieces are sold as lapidary specimens. It has modest collector value unless the mineral-filled pattern is distinctive.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the ocean floor. Rounded edges indicate natural water transport or deliberate tumbling.
Field identification & locations
Identify it by its heavy, fine-grained dark matrix, lack of visible large crystals, and possible gas cavities; a streak, acid, and hardness test can help distinguish calcite or quartz infill. Common sources include volcanic regions, river gravels, and beaches.
