igneous
Vesicular basalt
Basalt, a mafic volcanic rock composed chiefly of plagioclase feldspar, pyroxene, and commonly olivine
AI-generated identification · may be incorrectDark gray to black, fine-grained, with abundant rounded gas vesicles; typically dull to slightly vitreous. Hardness about 5–6 Mohs, no obvious cleavage, generally massive, and specific gravity about 2.7–3.0; the sparkling points may be zeolite or other cavity minerals.
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Physical properties
Dark gray to black, fine-grained, with abundant rounded gas vesicles; typically dull to slightly vitreous. Hardness about 5–6 Mohs, no obvious cleavage, generally massive, and specific gravity about 2.7–3.0; the sparkling points may be zeolite or other cavity minerals.
Formation & geological history
Formed when low-silica basaltic lava cooled rapidly at or near Earth’s surface, trapping bubbles; commonly Cenozoic to modern, though basalt occurs throughout geologic time. Vesicular textures are typical of lava flows, scoria, and volcanic edges.
Uses & applications
Used as crushed aggregate, roadstone, riprap, and decorative stone; attractive polished pieces are sold as geological specimens, though this example appears more natural than gem-grade.
Geological facts
Vesicles record gas exsolution during eruption, and later groundwater may fill them with zeolites, calcite, or quartz. It is denser and harder than pumice and usually lacks the reddish, highly frothy appearance of scoria.
Field identification & locations
Identify it by its black fine-grained matrix, abundant bubble cavities, and lack of visible large crystals; test density and hardness, and inspect cavities for secondary minerals. Common worldwide in volcanic regions, including Hawaii, Iceland, India, and the Columbia River area.
