igneous
Polished brown basalt or basaltic pebble
Basalt; fine-grained mafic volcanic rock, commonly composed of plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark brown to black, fine-grained, massive, and weakly lustrous from polishing; hardness about 5–6 Mohs, with no visible cleavage and typically specific gravity 2.8–3.0. The rounded, smooth form indicates water wear or intentional tumbling/polishing.
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Physical properties
Dark brown to black, fine-grained, massive, and weakly lustrous from polishing; hardness about 5–6 Mohs, with no visible cleavage and typically specific gravity 2.8–3.0. The rounded, smooth form indicates water wear or intentional tumbling/polishing.
Formation & geological history
Basalt forms when mafic lava rapidly cools at or near Earth’s surface, commonly during Cenozoic to modern volcanic activity, though basalt occurs throughout geologic time. The specimen may have weathered brown through oxidation of iron-bearing minerals.
Uses & applications
Used as aggregate, road stone, paving, railroad ballast, and dimension stone; polished pieces are sold as decor, worry stones, or lapidary specimens. Common material usually has modest commercial value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of oceanic crust. Its brown surface can obscure the original black color, while tiny pale specks may be plagioclase crystals or weathering products.
Field identification & locations
Look for a dense, fine-grained texture, dark color, heavy weight, and occasional tiny feldspar or vesicle features; it should not fizz with dilute acid. Common worldwide, especially volcanic regions and river gravels; exact identification would benefit from a fresh surface or microscope.
