Ice Diamond Riddle SOLUTION ft. Vsauce's Michael Stevens

Water is interesting as it cools because it expands as it cools just before it freezes but then contracts a bit just before it forms ice. So there is a theoretical case in an insulated container where if the ice and water are at the right quantities and temperatures the water will rise as the ice melts. This effect is not shown in most tables of expansion for water because they are not detailed enough. Putting a substantial diamond in the ice cube will overwhelm this effect for all practical purposes, so the water level will always fall.