igneous
Dark gray basalt or andesite cobble
Fine-grained mafic to intermediate volcanic rock; commonly plagioclase feldspar, pyroxene, and possibly olivine
AI-generated identification · may be incorrectDark gray, fine-grained, dull to slightly glassy luster, with indistinct crystals and irregular fracture; typical hardness is about 6 Mohs and specific gravity 2.7–3.0. The pale patches may be weathering, feldspar, or adhering mineral material.
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Physical properties
Dark gray, fine-grained, dull to slightly glassy luster, with indistinct crystals and irregular fracture; typical hardness is about 6 Mohs and specific gravity 2.7–3.0. The pale patches may be weathering, feldspar, or adhering mineral material.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface; its exact geological age and whether basalt or andesite cannot be determined from the photograph. Common in volcanic terrains and as rounded stream or beach cobbles.
Uses & applications
Used as crushed aggregate, road stone, railroad ballast, riprap, and building stone; attractive polished pieces may be used decoratively. This ordinary specimen has little commercial or jewelry value.
Geological facts
Basalt is the most widespread volcanic rock and forms much of the oceanic crust. Andesite is compositionally intermediate and is especially common in volcanic arcs.
Field identification & locations
Look for a dense, fine-grained texture, dark color, lack of obvious layering, and possible tiny feldspar crystals; a streak, hardness, magnetism, and acid test would improve identification. Common worldwide in volcanic regions and glacial or river gravels.
