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

Whether the ice floats, is equally buoyant as the water or sinks its total weight will be equal no matter if it is ice + diamond or water + diamond. Its density though will always decrease when the ice melts. It will always decrease to that less of water and since the diamond will always end up submerged it will always displace less water that what it did. So, level goes down.