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

Gonna take a wild guess here:
1 - We know that the ice is less dense than water, therefore ice with the same weight as water, has a higher volume
2 - We don't know the volume of the ice: 
     2.1 - So if we have ice that's "just about right" to cover the diamond, it will fall all down the cup and when the ice melts, since it wasn't a significant amount of ice, and the diamond is already full under the water (stabilized), it will stay the same.
     2.2 - If the ice if big enough compared to the volume of water we have inside the cup, it will sink full with the diamond, and when it melts, since the volume of a cube of ice is less than the volume of the water, the overall volume of the cup will decrease.
     2.3 - (This one I'm not so sure though) If the ice is just about right so that the cube of ice with the diamond inside floats on the top level of the cup, than the volume of the cup will stay the same (?).
    Those are the three points I can think of; One witch the ice is so insignificant compared to the overall volume of itself+diamond, than they will sink and volume stays the same after it melts, because there wasn't a significant amount of frozen water. One with two other small branches: (a) The ice is big enough to float and cover the diamond, thus when it melts, it will increase the volume of the cup (b) The ice is so big, that it will or go right down/stay on the middle of the cup, thus when the ice melts, like the first option, the volume will decrease.

Honestly not sure if I got any of this right, but I would like some feedback and a discussion so I can understand more about hydrodynamics, since I just started my physics degree :)