igneous
Polished basalt or basaltic river pebble
Basalt; fine-grained plagioclase–pyroxene rock, possibly containing olivine
AI-generated identification · may be incorrectHardness about 6–7 Mohs; dark brown to black, very fine-grained, with a smooth glassy-to-waxy luster from polishing. Basalt has no visible cleavage and commonly has compact massive texture; the brown area may be weathered iron-rich material or surface coating.
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Physical properties
Hardness about 6–7 Mohs; dark brown to black, very fine-grained, with a smooth glassy-to-waxy luster from polishing. Basalt has no visible cleavage and commonly has compact massive texture; the brown area may be weathered iron-rich material or surface coating.
Formation & geological history
Basalt forms when mafic lava cools rapidly at or near Earth’s surface, commonly in volcanic fields and oceanic crust; its age may range from recent to billions of years. The rounded shape indicates abrasion in a river, beach, or tumbling process.
Uses & applications
Used as aggregate, riprap, paving stone, and decorative stone; polished pieces are inexpensive curios or paperweights rather than typical jewelry gemstones.
Geological facts
The specimen appears human-polished or naturally water-smoothed, and the strong color boundary could indicate a basalt pebble with weathered rind, a coating, or a composite stone. Basalt is among Earth’s most abundant volcanic rocks.
Field identification & locations
Identify by its high density, dark color, fine texture, and lack of fizz in dilute acid; a streak test is usually gray-black. Common worldwide, especially volcanic regions and stream gravels; exact identification requires a fresh surface or microscopy.
