Multiple choice general knowledge --------- is the only rock that floats on water? sand rock pumice purnice rock quicksalt rock Reveal answer Fill a bubble to check yourself B Correct answer Explanation Pumice is a volcanic rock formed from frothy lava that rapidly cools, trapping numerous gas bubbles which make it less dense than water. This unique structure allows pumice to float, unlike sand rock, quicksalt rock, or purnice rock (a misspelling).