igneous
Hollow geode or vesicular volcanic rock
Likely quartz-lined basaltic/andesitic vesicle; exact identification requires testing
AI-generated identification · may be incorrectGray-brown, irregularly rounded specimen with a large cavity and a small pale lining that may be quartz or chalcedony; likely glassy to dull luster. Estimated hardness is 6–7 if quartz/chalcedony is present; crystal structure cannot be confirmed visually.
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Physical properties
Gray-brown, irregularly rounded specimen with a large cavity and a small pale lining that may be quartz or chalcedony; likely glassy to dull luster. Estimated hardness is 6–7 if quartz/chalcedony is present; crystal structure cannot be confirmed visually.
Formation & geological history
Probably formed when gas bubbles became trapped in cooling lava, later developing silica minerals along the cavity walls. Such vesicular volcanic rocks are generally Cenozoic to recent, depending on locality.
Uses & applications
Primarily a decorative or educational specimen; quartz-lined cavities may have modest collector value, but it is not suitable for structural construction.
Geological facts
The cavity is a vesicle, and its mineral lining can distinguish an ordinary volcanic vesicle from a true geode. A metal streak test, hardness test, and close inspection for crystals would improve identification.
Field identification & locations
Common in basaltic volcanic regions, including the western United States, Mexico, Iceland, India, and parts of Europe. Quartz should scratch glass but not be scratched by a steel knife; avoid testing on polished surfaces.
