igneous
Rounded basalt cobble
Basalt; fine-grained mafic volcanic rock, commonly containing plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray-green, dull to weakly earthy luster, very fine-grained and massive, with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. Its smooth, rounded shape indicates prolonged water or sediment transport.
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Physical properties
Dark gray-green, dull to weakly earthy luster, very fine-grained and massive, with no visible cleavage; Mohs hardness about 5–6 and specific gravity roughly 2.8–3.0. Its smooth, rounded shape indicates prolonged water or sediment transport.
Formation & geological history
Basalt forms when iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in oceanic crust, volcanic fields, and flood-basalt provinces. The specimen’s exact age and locality cannot be determined from the image.
Uses & applications
Used as crushed aggregate, road base, railroad ballast, riprap, and occasionally building stone; attractive cobbles are used in landscaping and collections. This ordinary specimen has little commercial gemstone value.
Geological facts
Basalt is the most abundant volcanic rock and makes up much of the ocean floor. Vesicles, if present, may later be filled by zeolites, calcite, or quartz.
Field identification & locations
Identify it by its dense, fine-grained dark gray appearance, lack of obvious crystals, and resistance to scratching; a steel nail may leave little or no mark. Common sources include streambeds, volcanic terrains, beaches, and glacial deposits.
