From the Super Mario Wiki
The Diamond Star is the first Crystal Star, which Koopley, Koops' father, found in Hooktail's belly in the game Paper Mario: The Thousand-Year Door. This Crystal Star will teach Mario the move Earth Tremor, which costs two SP. To use this move, the player must press the button when a red light illuminates. Doing this perfectly can result in a shockwave so large, it will hit enemies on the ceiling, such as Swoopers. It can cause up to six HP damage to all enemies and ignores defense.