Ice Diamond Riddle SOLUTION ft. Vsauce's Michael Stevens

The way I envisaged was the combined weight of the icecube and diamond was such that the entire icecube was floating but level with the top of the water so the entire volume of the cube was underwater. Once it melts the cube of water takes up less volume than the cube of ice did since it's more dense and so the water level would drop slightly since the cube of water plus the diamond displaces less then the cube of ice plus the diamond