Geodes are formed when there are pockets of air within rocks. This often happens after volcanic eruptions when lava cools around air bubbles. These pockets leave space for groundwater to seep in. But the water itself doesn't produce geodes–it brings along minerals which stay in the rock even after the water evaporates.

How long does it take for geodes to form?

How long does it take for a geode to form? Over thousands of years, these layers of minerals build crystals that eventually fill the cavity. How long this takes depends on the size of the geode. The largest crystals can take a million years to grow!

What kind of rock do geodes form in?

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lava; or, as in the American Midwest, in rounded cavities in sedimentary formations.