THE WONDER OF PUMICE ROCK Not all rocks sink, some float. Pumice is a volcanic rock formed when hot lava full of gas cools quickly and hardens. The fast cooling traps many tiny air pockets inside, making the rock lighter than water. That is why it floats for months or even years before sinking. Pumice is more than a fun science fact, it has many uses too. It is used for skin care, water filtering, and in lightweight building materials. This floating rock shows the power and creativity of nature.