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

It remains the same. By the principle of flotation the upthrust on the ice+diamond=it's combined weight. By Archimedes principle, the upthrust experience is equal to the weighr of water displaced. As the ice melts it loses mass and hence weight, the upthrust on it decreases and the amount of water it displaces decreases causing the water level to fall. However as the ice turns into water, the amount of water in the cup increases and raises the water level by an equal amount thus the water level remains the same. As the diamond is frozen in the ice cube, the weight is already considered in the calculation of upthrust. So when the ice cube melts and releases the diamond the amount of water is displaces is the same. Thus overall, the water level remains the same.