igneous
Polished brown basalt (likely vesicular basalt)
Basalt; chiefly plagioclase, clinopyroxene, and iron-rich minerals, with possible weathering rind
AI-generated identification · may be incorrectDark brown to black, fine-grained, locally vesicular, with dull to subtly glassy luster; hardness about 5–6 Mohs and no obvious cleavage. The rounded form and smooth surface indicate water or abrasion polishing.
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Physical properties
Dark brown to black, fine-grained, locally vesicular, with dull to subtly glassy luster; hardness about 5–6 Mohs and no obvious cleavage. The rounded form and smooth surface indicate water or abrasion polishing.
Formation & geological history
An extrusive igneous rock formed when mafic lava cooled rapidly at or near Earth’s surface, generally in the Cenozoic or older volcanic terrains. Brown coloration commonly results from oxidation and weathering of iron-bearing minerals.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a polished decorative or lapidary specimen. It has little gemstone value unless unusually patterned or exceptionally polished.
Geological facts
Basalt is the most widespread volcanic rock on oceanic crust and commonly forms lava flows, dikes, and seafloor pillow lavas. The apparent pale crust may be weathering, adhering sediment, or mineral coating rather than a separate rock type.
Field identification & locations
Identify it by its heavy feel, dark fine-grained matrix, lack of visible large crystals, and weak attraction to a magnet if magnetite is present; a streak test is usually gray-black. Common worldwide, especially volcanic regions and river or beach gravels.
