igneous
Weathered basalt cobble
Basalt; fine-grained plagioclase–pyroxene volcanic rock, commonly with olivine
AI-generated identification · may be incorrectDark reddish-brown, dull to weakly earthy luster, rounded by transport and weathering, with small vesicles or pits; hardness about 5–6 Mohs, no visible cleavage, very fine-grained crystalline texture, and typical specific gravity around 2.8–3.0.
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Physical properties
Dark reddish-brown, dull to weakly earthy luster, rounded by transport and weathering, with small vesicles or pits; hardness about 5–6 Mohs, no visible cleavage, very fine-grained crystalline texture, and typical specific gravity around 2.8–3.0.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near Earth’s surface, likely in the Cenozoic or older depending on locality; the red-brown color reflects oxidation of iron-bearing minerals during weathering.
Uses & applications
Used as aggregate, road stone, riprap, and occasionally landscaping material; attractive polished pieces may be sold as inexpensive display specimens.
Geological facts
Basalt is the most widespread volcanic rock and forms much of oceanic crust; iron oxidation can make otherwise black basalt appear brown, red, or rust-colored.
Field identification & locations
Identify by its heavy weight, fine texture, dark iron-rich color, and lack of obvious mineral grains; common in volcanic regions, river gravels, beaches, and glacial deposits. The image alone cannot confirm basalt versus weathered andesite or iron-rich chert.
