igneous
Possible basalt or basaltic volcanic rock
Basalt; typically plagioclase feldspar, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, irregular and locally glassy-looking with a subvitreous to dull luster; usually hardness 5–6 Mohs, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The glossy surface may reflect weathering or a thin coating, so exact identification from the image is uncertain.
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Physical properties
Dark gray to black, fine-grained, irregular and locally glassy-looking with a subvitreous to dull luster; usually hardness 5–6 Mohs, massive with no obvious cleavage, and specific gravity about 2.8–3.0. The glossy surface may reflect weathering or a thin coating, so exact identification from the image is uncertain.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly from volcanic flows; specimens may range from recent to Precambrian age.
Uses & applications
Widely used as crushed aggregate, road base, railroad ballast, concrete material, and dimension stone; attractive polished pieces are sometimes sold as decorative stone.
Geological facts
Basalt is the most common volcanic rock and forms much of the oceanic crust. Vesicles, if present, may later contain zeolite, calcite, or quartz minerals.
Field identification & locations
Look for a dense black fine-grained texture, possible tiny pale feldspar crystals, and a hard surface that resists scratching by a steel knife. Common locations include volcanic regions, oceanic islands, and basalt plateaus.
