igneous
Weathered basalt cobble
Basalt, a fine-grained mafic volcanic rock
AI-generated identification · may be incorrectTypically 5–6 Mohs; dark gray to brown with a dull to faintly vitreous luster, very fine interlocking crystals, and no visible cleavage. The rusty orange coating is likely iron-oxide weathering (limonite/goethite).
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Physical properties
Typically 5–6 Mohs; dark gray to brown with a dull to faintly vitreous luster, very fine interlocking crystals, and no visible cleavage. The rusty orange coating is likely iron-oxide weathering (limonite/goethite).
Formation & geological history
Formed when iron- and magnesium-rich lava rapidly cooled at or near Earth’s surface, commonly in the Cenozoic or older volcanic terrains; the rounded shape indicates stream or beach abrasion.
Uses & applications
Used as aggregate, roadstone, riprap, landscaping stone, and occasionally as a polished decorative material; this small cobble has mainly educational or modest collecting value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of oceanic crust. Its iron-rich minerals commonly weather to reddish-brown surfaces.
Field identification & locations
Identify by its heavy, dense feel, dark fine-grained interior, lack of obvious quartz, and possible weak attraction to a magnet. Common in volcanic regions, river gravels, and beaches; a fresh broken surface would confirm the texture.
