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

It goes down, if it was pure ice the volume would stay the same because the volume displaced to float would be the same added by the melted ice, so you only need consider the diamond. The diamond would displace 3.5 times its own volume of water when inside the floating ice but only 1 time when sunk, the total volume that makes the water level would decrease 2.5 diamond sizes making the level goes down.