igneous
Rounded basalt cobble
Basalt, predominantly plagioclase feldspar and pyroxene with minor olivine and iron oxides
AI-generated identification · may be incorrectDark brown to gray, fine-grained and massive, with dull to weakly vitreous luster; hardness about 5–6 Mohs, generally no visible cleavage, and specific gravity around 2.8–3.0. The smooth shape indicates water or glacial abrasion; pale patches may be weathered minerals or attached sediment.
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Physical properties
Dark brown to gray, fine-grained and massive, with dull to weakly vitreous luster; hardness about 5–6 Mohs, generally no visible cleavage, and specific gravity around 2.8–3.0. The smooth shape indicates water or glacial abrasion; pale patches may be weathered minerals or attached sediment.
Formation & geological history
An extrusive igneous rock formed when basaltic lava cooled rapidly at or near Earth’s surface, commonly during Cenozoic or older volcanic activity. Its rounded form likely developed in a river, beach, or glacial setting.
Uses & applications
Used as aggregate, road base, riprap, landscaping stone, and occasionally as a polishing or carving material; ordinary cobbles have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and forms much of the oceanic crust. Vesicles may occur, but this specimen appears mostly dense and non-vesicular.
Field identification & locations
Identify it by its dark, fine-grained appearance, substantial weight, and lack of obvious layers or large crystals; a streak test is usually gray, and it generally does not fizz with dilute acid. Common worldwide in volcanic terrains and river gravels.
