"You got the Mysterious Crystals! This mass is made up of faintly glowing square crystals. Several are missing."
— In-game description

The Mysterious Crystals are an item from The Legend of Zelda: Skyward Sword. They are used as a key to enter the boss room of the Fire Sanctuary, in which Ghirahim is fought for the second time in the game. In order to open the door leading to the dungeon's boss, Link must correctly maneuver the crystals so that they fit into the lock.

See also