igneous
Vesicular basalt (likely scoriaceous basalt)
Basalt, predominantly plagioclase–pyroxene–olivine; approximate composition 45–52% SiO₂
AI-generated identification · may be incorrectDark gray to black, fine-grained and strongly vesicular, with a dull to locally submetallic luster; hardness about 5–6 Mohs, no prominent cleavage, and typical specific gravity around 2.6–2.9 (lower if highly vesicular).
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Physical properties
Dark gray to black, fine-grained and strongly vesicular, with a dull to locally submetallic luster; hardness about 5–6 Mohs, no prominent cleavage, and typical specific gravity around 2.6–2.9 (lower if highly vesicular).
Formation & geological history
Formed when low-viscosity basaltic lava cooled rapidly at or near the surface, trapping gas bubbles; commonly Cenozoic to recent, though basalt occurs throughout geological time.
Uses & applications
Used as aggregate, road stone, riprap, and landscaping rock; vesicular pieces are commonly collected as volcanic specimens, though this example is not typically jewelry grade.
Geological facts
The rounded cavities are vesicles formed by escaping volcanic gases; some may later be lined or filled with zeolites, calcite, or quartz.
Field identification & locations
Identify it by its black fine-grained groundmass, irregular gas cavities, and relatively heavy weight; common in volcanic regions such as Hawaii, Iceland, the Cascades, East Africa, and oceanic islands. The image could also represent vesicular scoria, but its dense, dark surface favors basalt.
