geode - teaching hand specimen of a typical unopened geode
Geodes are bubble fillings that weather out of a vesicular lava. As the flow cools, unescaped gases form voids in the otherwise solid flow. Subsequent hydrothermal activity fills these voids with mineral-rich hot water and as it cools, crystals grow on the surface of the roughly spherical opening. In the lava flow, the immediate zone around the now crystal lined bubble is altered and becomes more resistant than the surrounding rock and subsequently weathers out. Often they are completely mineral-filled. Some are hollow.
These geodes are best left unopened, as the quartz crystal lining is often unspectacular, and these get across to students that they are bubble-fillings. They look bubbly. We don't have locality data other than "Mexico."
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