igneous
Weathered basalt cobble
Basalt; fine-grained mafic volcanic rock, chiefly plagioclase and pyroxene with possible olivine
AI-generated identification · may be incorrectTypically Mohs 5–6, dark gray to brown with dull/earthy luster and very fine interlocking crystals; no visible cleavage, irregular fracture, and moderate-to-high density (~2.8–3.0 g/cm³). The rusty-brown surface reflects iron oxidation and weathering.
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Physical properties
Typically Mohs 5–6, dark gray to brown with dull/earthy luster and very fine interlocking crystals; no visible cleavage, irregular fracture, and moderate-to-high density (~2.8–3.0 g/cm³). The rusty-brown surface reflects iron oxidation and weathering.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly in volcanic flows or oceanic crust; the cobble was later rounded and abraded by water or wave action. Basalt ranges from Precambrian to modern in age.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally dimension stone; ordinary cobbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. This specimen’s rounded shape and weathered rind indicate transport and surface exposure.
Field identification & locations
Identify by its heavy, dark, fine-grained appearance, lack of obvious layering, and possible tiny pale plagioclase flecks; a streak, acid, or thin-section test would improve certainty. Common worldwide, especially volcanic islands, river gravels, and beaches.
