igneous
Vesicular basalt pebble
Basalt; chiefly plagioclase, pyroxene, and olivine
AI-generated identification · may be incorrectDark gray to brown, fine-grained, dull to subtly vitreous, with small vesicles and weathered iron-oxide staining; hardness about 5–6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0).
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Physical properties
Dark gray to brown, fine-grained, dull to subtly vitreous, with small vesicles and weathered iron-oxide staining; hardness about 5–6 Mohs, no visible cleavage, typically dense (specific gravity ~2.8–3.0).
Formation & geological history
An extrusive volcanic rock formed when mafic lava rapidly cooled at or near Earth’s surface; its exact age cannot be determined from the image and may range from recent to billions of years.
Uses & applications
Used as crushed aggregate, road metal, riprap, ballast, and dimension stone; attractive polished pieces are collected but it is generally not a gemstone.
Geological facts
The holes are gas bubbles preserved as vesicles, while brown coloration commonly reflects oxidation of iron-bearing minerals during weathering.
Field identification & locations
Common in volcanic regions and river or beach gravels, including Hawaii, Iceland, the Pacific Northwest, and oceanic islands. Identify it by its dark fine-grained matrix, vesicles, high density, and lack of obvious quartz crystals; a streak test is usually gray to black.
