As Inno once explained (revealed), when a bug hit the game that affected diamonds. They are of course carefully monitored, and are from their own server/service.
In fact, students of the game can accurately and repetitively guess correctly about your GbG reward earned when diamonds are won because... there is a palpable but very short "blip" (tiny pause) when the reward is revealed to you for the advancement you just made that probably comes from the extra accounting/servicing of true game currency (diamonds) vs things like coins or supplies. You can get pretty good and correct before the award message box is revealed. - with practice, of course, and over time you play GbG.
"oh wow, I am about to be notified that I won some diamonds!"
In GE too, same paradigm. Slight pause when you win diamond rewards for encounters.