Stunning Oregon Thunderegg Agate - Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure.
* I think these are man made* Fulgurite Sphere. Also known as Lightning Glass, these delicate, root-like formations are created when lightning strikes rock, instantly melting and fusing the stone into a preserved model of the lightning.