Geodes form when hydrothermal fluids fill bubbles in a solidified lava flow, slowly depositing a mineral crust on the inside of the bubble. Subsequent waves of hydrothermal mineralization will either completely fill the cavity or result in a lining of mineral crystals. When the lava weathers and erodes, the more resistant geodes are left behind. Our geodes are unopened for the most part and look like bubbles, reinforcing the concept for students.
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