From the Super Mario Wiki
The Garnet Star is the sixth Crystal Star, found at Pennington's home, the Poshley Sanctum in the game Paper Mario: The Thousand-Year Door. This was hidden in the painting at the Sanctum for safety, while a fake one was put out in the open to fool any would-be thieves, including the Shadow Sirens, Bowser, and Kammy Koopa.
This Crystal Star will teach Mario the move Showstopper, which costs two SP. To use this move, the buttons must be pressed in the order that they appear on the screen. If used correctly, it may defeat all on-screen enemies with one blow; however it does not work on bosses.