igneous
Vesicular basalt (possibly scoriaceous basalt)
Basalt; chiefly plagioclase feldspar, pyroxene, and commonly olivine, with volcanic glass
AI-generated identification · may be incorrectDark gray to black, fine-grained, strongly vesicular and weathered, with dull to subvitreous luster; hardness about 5–6 Mohs, no prominent cleavage, and typical specific gravity around 2.7–3.0.
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Physical properties
Dark gray to black, fine-grained, strongly vesicular and weathered, with dull to subvitreous luster; hardness about 5–6 Mohs, no prominent cleavage, and typical specific gravity around 2.7–3.0.
Formation & geological history
An extrusive mafic igneous rock formed when low-silica lava cooled rapidly at or near the surface, trapping gas bubbles. It may range from recent volcanic material to rocks hundreds of millions of years old; the image alone cannot establish its age.
Uses & applications
Used as aggregate, road base, riprap, ballast, and occasionally dimension stone; attractive vesicular pieces are collected but are generally not jewelry-grade.
Geological facts
The abundant cavities are vesicles formed by escaping volcanic gases; later mineral growth can fill them to produce amygdaloidal basalt.
Field identification & locations
Identify it by its black mafic color, fine texture, irregular vesicles, and substantial weight; it is common in volcanic regions, lava flows, and basalt-rich gravel. A fizzing reaction with acid would suggest carbonate infill rather than fresh basalt.
