igneous
Basalt pebble with vesicle
Basalt; predominantly plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectFine-grained, dark gray to black, matte to weakly silky luster, with a small gas cavity (vesicle) containing possible secondary mineralization. Hardness about 6–6.5 Mohs; typically massive, with no visible cleavage; specific gravity about 2.8–3.0.
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Physical properties
Fine-grained, dark gray to black, matte to weakly silky luster, with a small gas cavity (vesicle) containing possible secondary mineralization. Hardness about 6–6.5 Mohs; typically massive, with no visible cleavage; specific gravity about 2.8–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when iron- and magnesium-rich lava cooled rapidly at or near Earth’s surface, commonly during the Cenozoic or older volcanic episodes. The cavity formed from trapped gas bubbles.
Uses & applications
Used as aggregate, road base, riprap, railroad ballast, and building stone; attractive polished pieces are collected but usually have modest value.
Geological facts
Basalt is Earth’s most abundant volcanic rock and forms much of the oceanic crust. Vesicles may later fill with zeolite, calcite, quartz, or other minerals.
Field identification & locations
Identify it by its uniformly dark, fine-grained appearance, substantial weight, and lack of fizz with dilute acid; a streak is typically gray. Common along volcanic coasts, river gravels, and lava fields.
