The only type of rock that will consistently float on water is pumice.

Here's why:

* Pumice is a volcanic rock formed from frothy lava. This process creates a very porous structure with lots of air pockets.

* The air trapped inside the pumice makes it less dense than water. This is the key to flotation.

While other rocks might occasionally float if they have a unique shape that traps air, pumice is the only rock type guaranteed to float because of its inherent structure.