igneous
Weathered basalt or mafic volcanic rock
Basalt; chiefly plagioclase feldspar, pyroxene, and minor olivine/iron oxides
AI-generated identification · may be incorrectFine-grained, dull earthy brown from weathering, with possible tiny dark crystals; typically massive and nonvesicular in this specimen. Fresh basalt has Mohs hardness about 6, no cleavage as a rock, and specific gravity around 2.8–3.0.
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Physical properties
Fine-grained, dull earthy brown from weathering, with possible tiny dark crystals; typically massive and nonvesicular in this specimen. Fresh basalt has Mohs hardness about 6, no cleavage as a rock, and specific gravity around 2.8–3.0.
Formation & geological history
Forms when low-silica lava cools rapidly at or near Earth’s surface, commonly in oceanic crust and volcanic fields; specimens may range from geologically recent to billions of years old. The brown coating reflects oxidation and surface alteration.
Uses & applications
Used as crushed aggregate, road base, riprap, railroad ballast, and building stone; unremarkable weathered pieces have little jewelry value.
Geological facts
Basalt is the most abundant volcanic rock on Earth and makes up much of the ocean floor. Its dark minerals commonly oxidize to brown or reddish hues during weathering.
Field identification & locations
Identify by its heavy, fine-grained texture, dark original color, and lack of visible layering; a streak, hardness, and acid test can help distinguish it from sandstone or iron-rich concretions. Common in volcanic regions and glacial gravels.
