Ice Diamond Riddle SOLUTION ft. Vsauce's Michael Stevens

So, as is often the case, thinking about it more results in improving the answer.

While, yes, the water level would ultimately be at equilibrium lower than when the ice cube is floating, it would not be a sudden transition. Rather what happens is that at some point the cube will shift from floating, to not floating. That is, at some point the density of the ice cube and diamond will precisely match that of the water. Now the exact point would depend on how large the diamond is, but using your value of 3.5 g/cm^3 it will be roughly when the volume of ice is 31.25 the volume of the diamond.

It seems high, but that’s because ice has just over 90% the density of water. It’s buoyant, but it can only support a small amount of denser material. So one cm^3 of diamond (3.5g) would require 31.25 cm^3 of ice to be neutral buoyancy, for a total volume of 32.25 cm^3.

But it’s not like the ice would melt all at once. Once it hits this threshold, it would continue to melt. Increasing the density of the cube slowly, over time. So it would sink at this point, and continue to shrink. However since the density keeps increasing (as volume decreases) the water level would drop over time, in roughly linear fashion, until all the ice is melted and the new equilibrium is reached.

So there is no sudden shift, just continuous change from one equilibrium state, to another equilibrium state.