Not sure about diamonds specifically, but a single diamond ring is hardly anything when compared to the camels laden with all sorts of jewlery we find eliezer bringing to rivka when doing a shidduch for yitzchak.
The mishna in kiddushin regarding erusin is also pretty clear, kessef OR shava kessef. I don’t have the daf right now but in the sugya of “trichi shuma” there is a tosfos that discusses whether one should use precious stones for nissuin or not and concludes for nissuin a plain band is customary, nobody has to be a baki how much it is worth. Perhaps that is why the expensive stuff was reserved for the erusin (our engagement today _ is sort of the erusin of those days) where “bekiin bishuma” is not a factor. Just a guess. I’m sure the CR poskim will be out in full force a bit later to clarify for everyone.