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

It's the stone in the canoe riddle reformatted (my favorite physics riddle, BTW): While floating in the ice, the ice and the diamond will displace an amount of water equal to their combined weight (buoyant force). The melting ice and the water are equal in mass, so the amount of water displaced while floating is equal to its volume when liquid. However, when the diamond sinks, it will only displace its volume. Since it is more dense, the level of the water will be very slightly lower than when it was floating. (It also depends on if the diamond is heavy enough to submerge the ice cube...)