The diamond egg is the second star rewards egg which you can only obtain after the golden egg. The Star rewards are the same but instead of a golden egg, you will receive a diamond egg. No body in the game has one yet. You would need a 360 day login streak to obtain this egg. The eggs appearance is a plain egg but a shiny diamond blue color. The three legendary pets you can get (with a one in three chance) are: the diamond griffin, the diamond dragon and the diamond unicorn. You would need 1,320 stars to get this.