Here's why:

* Geodes are geological formations that are typically round or oval in shape. They are formed when cavities in volcanic rock or sedimentary rock are lined with crystals.

* Limestone is a sedimentary rock, so it can form geodes.

* The crystals inside a geode often form a hollow sphere, giving it a unique appearance.

While the term "geode" is most commonly used, sometimes you might hear them called "limestone geodes" or "geodes in limestone" to emphasize the specific type of rock they are found in.