The Crimson Diamond is a key item that appears in Wario: Master of Disguise.
The Crimson Diamond is what it's name states: a diamond that is crimson in color.
The Crimson Diamond is a very important item that opens the door to the final room of Allergia Gardens where the final piece of the Wishstone is held as well as the demon sealed within it: Terrormisu. After Carpaccio beats Wario to Allergia Gardens, he takes the Crimson Diamond and hides it away hoping Wario wouldn't find it and unintentionally release Terrormisu. However, after navigating through the enormous maze of this level, Wario was able to find the Crimson Diamond and make it through the door with the Wishstone piece.