Pumice is a type of rock that can actually float. It is an igneous rock formed by explosive volcanic eruptions, and is so porous that it floats until the pores fill and become waterlogged.
The pumice rock is formed when a rush of gas infuses the magma as it blows from a volcano under great force. The gas creates bubbles that are trapped in the rock as it cools rapidly. The abundant pores, or vesicles, thereby created give pumice a specific gravity lower than one, allowing pumice to float on water. Pumice has been known to float for years before sinking, sometimes as large pumice "rafts" created by underwater or island volcanic eruptions.