Geodes are created in the hollow areas of soil such as animal burrows or tree roots. They are also formed in the bubbles in volcanic rock.

Over time, dissolved minerals seep into a hollow area and harden into an outer shell creating the geode. The minerals continue to form on the inside walls of the shell, growing towards the center. The most common dissolved mineral is quartz, but amethyst and other minerals are also found.

It can take hundreds of millions of years for the space inside a geode to be filled, and many geodes remain partly hollow. A geode which is completely filled with crystals is called a nodule. Agate-filled nodules are called thunder eggs.