igneous
Weathered basaltic cobble
Basalt, likely amygdaloidal or vesicular basalt
AI-generated identification · may be incorrectDark gray-brown, fine-grained, dull to weakly earthy luster, with weathering pits and possible altered vesicles; chiefly plagioclase and pyroxene, sometimes olivine. Hardness about 5–6 Mohs; massive, lacking visible cleavage; specific gravity roughly 2.8–3.1.
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Physical properties
Dark gray-brown, fine-grained, dull to weakly earthy luster, with weathering pits and possible altered vesicles; chiefly plagioclase and pyroxene, sometimes olivine. Hardness about 5–6 Mohs; massive, lacking visible cleavage; specific gravity roughly 2.8–3.1.
Formation & geological history
Formed when mafic lava rapidly cooled at or near Earth’s surface, commonly in volcanic flows. Its rounded shape indicates stream, beach, or glacial transport, while the brown surface reflects oxidation and chemical weathering; age cannot be determined from the image.
Uses & applications
Used as aggregate, road base, riprap, and dimension stone; attractive polished pieces may be collected, but this specimen has little jewelry value.
Geological facts
Basalt is the most widespread volcanic rock on Earth and forms much of the oceanic crust. Vesicles may later be filled by minerals such as zeolite, calcite, or quartz.
Field identification & locations
Identify it by its heavy, fine-grained mafic texture, dark color, and lack of obvious quartz or large crystals; a streak, fresh broken surface, and magnet test would help confirm it. Common in volcanic regions, river gravels, and glacial deposits.
