igneous
Vesicular basalt (possibly amygdaloidal basalt)
Basalt, a mafic volcanic rock composed mainly of plagioclase feldspar, pyroxene, and commonly olivine; cavities may contain secondary zeolites, calcite, or quartz.
AI-generated identification · may be incorrectHardness about 6 on Mohs scale; dark brown to black, weathered rusty-brown, with a dull to subtly vitreous luster. The abundant rounded vesicles formed from gas bubbles; some appear filled with pale minerals (amygdules).
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Physical properties
Hardness about 6 on Mohs scale; dark brown to black, weathered rusty-brown, with a dull to subtly vitreous luster. The abundant rounded vesicles formed from gas bubbles; some appear filled with pale minerals (amygdules).
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping volcanic gases; likely Cenozoic or younger, though age cannot be determined from the image. Weathering has enlarged or accentuated some cavities.
Uses & applications
Used as aggregate, riprap, road stone, and occasionally decorative stone; attractive amygdaloidal specimens are collected, but this porous piece has limited jewelry value.
Geological facts
Its holes are vesicles, not sedimentary fossils. Pale cavity fillings can indicate zeolites, calcite, or quartz and may fluoresce or react weakly with acid if calcite is present.
Field identification & locations
Identify by its very fine-grained dark matrix, irregular gas cavities, heavy weight, and lack of layering; common in volcanic regions, including Hawaii, Iceland, India, and the Pacific Northwest. Check cavity fillings with a hand lens and test suspected calcite with dilute hydrochloric acid.
