Geodes are hollow rocks with crystals grown inside them! 

 Classic localities for geodes are found in Illinois, Iowa, Indiana and outside the US such as Mexico and Brazil. Most geodes are sedimentary, while thundereggs are volcanic. Many geodes have a single layer or multiple layers of agate or chalcedony, and then crystal formations such as quartz crystals, and you get to the center of the geode. Sometimes many different types of crystals will form as the chemistry in the interior changes. Secondary mineral crystals can include sphalerite, millerite, chalcedony, and different colors of agate layers. 

Thundereggs are usually solid with layers of color agate filling the egg.

Thundereggs are solid balls of rock that have chalcedony agate and sometimes crystals in side. Thundereggs are formed very differently than geodes. Classic localities for thundereggs are in Oregon, Utah, Washington state and Montana and South Dakota.  Most thundereggs are solid, but occasionally will have a small hollow that has space for crystal formations of secondary minerals.