igneous
Weathered basalt cobble
Basalt, a fine-grained mafic volcanic rock composed mainly of plagioclase, pyroxene, and commonly olivine
AI-generated identification · may be incorrectTypically dark gray to brown with dull/earthy luster, very fine crystalline texture, hardness about 5–6 Mohs, and no obvious cleavage; the rusty patches are likely iron-oxide weathering. The elongated shape appears naturally water-worn.
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Physical properties
Typically dark gray to brown with dull/earthy luster, very fine crystalline texture, hardness about 5–6 Mohs, and no obvious cleavage; the rusty patches are likely iron-oxide weathering. The elongated shape appears naturally water-worn.
Formation & geological history
Formed when low-silica lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows or oceanic crust; basalt ranges from Precambrian to modern in age. Subsequent stream or beach abrasion produced the smooth cobble.
Uses & applications
Used extensively as crushed aggregate, road base, concrete stone, riprap, and building stone; this specimen has mainly educational or modest decorative value.
Geological facts
Basalt is the most widespread volcanic rock and makes up much of the ocean floor. Its fine grain reflects rapid cooling, unlike coarse-grained gabbro, its intrusive equivalent.
Field identification & locations
Identify it by its dark, dense, fine-grained appearance, lack of visible large crystals, and possible attraction to a strong magnet if magnetite is present. Common worldwide in volcanic regions, riverbeds, and beaches; exact identification would benefit from a fresh broken surface or mineral test.
