geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Okay, so not all geodes are thunder eggs, but just because a thunder egg has a hollow doesn't preclude it from still being a thunder egg. I guess the geological structure or formation process is the key. Okay, here are some pictures of a geode and thunder egg? The thunder egg was found at NM Rockhound State Park. Yes they let you dig in a state park for rocks.
|I found this thunder egg in the park. It has a solid quartz center (Needs some polishing)|
|I found these nodules near the NM AZ border. I suspected they might be geodes.|
|Geode with Tabular quartz crystal|
|Geode with dirty center (chalcedony)|
|Geode? No void/vug|
|Geode with tinted green Chalcedony|
|These have a hollow inside with what appear to be chalcedony|