From the Super Mario Wiki
The Crimson Diamond, also known as the crimson gem, is a key item from Wario: Master of Disguise. It is used to open a large door in Allergia Gardens that leads to the final piece of the Wishstone. At the beginning of the final episode, Carpaccio steals the Crimson Diamond and hides it in a purple treasure chest in a remote area of the garden. To find it, Wario must explore the garden and find three colored keys. After obtaining the Crimson Diamond, Wario can use it to open the final door and face Terrormisu.