The volcanic rock known as pumice is the only rock that can float in water.
Pumice is lava that cools and hardens quickly after eruption. The holes that give pumice its distinctive texture are caused by air bubbles that are trapped as the lava changes state from liquid to solid. And it is those same air bubble pockets that allow pumice to float -- at least for a while. If left in water long enough, however, pumice will, over time, sink, as the air bubbles fill with water.
Check out the Science NetLinks Earth Science Week Collection for many great resources on volcanoes, rocks, and events that shape the earth.