It's not unusual to be asked in a quiz: "What's the only rock that floats on water?"
Most reputable sources are more circumspect. Wikipedia, for example, says "Pumice has an average porosity of 90%, and initially floats on water" (my italics). It goes on to say, "Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water."
© Haydn Thompson 2017