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

The level of water would probably fall.

How much the water level fall is depending on the ratio of the diamond to the ice cube, which (I guess...) the larger the diamond to the total volume of the "ice cube with diamond", the lower it affect the fall in water level, as original "ice cube with diamond" will displace more water (due to its higher average density to water) at the start while the "ice cube" slowly melts away, the displacement of the "ice cube" reduces which lower down the water level over time. (and probably need to cover the lid up because vaporization can occur)

This is like the "disappearance" of matter, although it actually isn't.

(well, Science~ /shrug)