Can You Solve this Ice Diamond Riddle? ft. Vsauce’s Michael Stevens

100% easy, it stays the same because - within the ice cube, the mass taken up by the diamond cannot be taken up by water and so the tiny area where the diamond's water would be is shifted elsewhere in the ice cube and so the total amount of water never changes, it might just shift slightly to accommodate for the convection currents caused by the diamond finally falling through the water (probably along with a small lump of ice as the diamond is more dense than ice)