Here's why:

* Density: Whether something floats or sinks depends on its density compared to the density of water.

* Geode Composition: Geodes are made of various minerals, and the density of those minerals can vary significantly. Some geodes are filled with lightweight minerals like quartz, while others contain denser minerals like calcite or even heavy metals.

* Shape and Size: The shape and size of a geode also affect its buoyancy. A large geode, even if it's made of relatively light minerals, might be too heavy to float.

In short, a geode will float if its average density is less than the density of water (approximately 1 gram per cubic centimeter).