Ice Diamond Riddle SOLUTION ft. Vsauce's Michael Stevens

I came up with the same answer by addressing the riddle slightly different... 

I made my diamond be heavy enough in comparison to the ice (ratio) that the diamond weight would sink the ice cube completely. 

This would then only leave the fact that we were told the ice cube will melt into water. As you both mentioned ice being less dense (more volume with the same weight) than it's counterpart liquid water (less volume with the same weight).

You would see the water level decrease after the solid ice melted into liquid water. 

Great video from both of you!