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

The water level in the glass AFTER the diamond/ice is added will go down as the ice melts.  If the diamond is proportionately large and the combination sinks immediately, the melting ice reduces in volume taking the water level down.  If the diamond is small enough that the combination floats, the volume of water displaced to float the diamond will be more than the diamond will displace once the ice melts and the diamond sinks...the water level will still go down.