The swirling, layered bands of chalcedony in a polished agate geode sphere create an otherworldly quality, broken dramatically by crystal-lined cavities and angular gaps. Typically formed as nodules within volcanic rock, agate geodes start as “stone bubbles” in gas pockets within rhyolite lava as it begins to cool. Also called a “thunderegg”, over time the minerals in a geode crystallize into hollow veins of quartz, amethyst, or gypsum.

Hillman Hall of Minerals and Gems
Carnegie Museum of Natural History
Pittsburgh, PA

WALKING THE 'BURGH WEEK
06.15.26