Diamond Key

The Diamond Key is a special item in Paper Mario: The Origami King. It is one of the many keys picked up by Luigi on his quest for the Key to Peach's Castle. As such, it is found in possession of Luigi, who resides on Mushroom Island at the time. Originally in a chest on Full Moon Island, it is replaced by a note from Luigi and the Mushroom Handle. Luigi is seen resting, satisfied, thinking he has finally found the key to Peach's Castle. However, it turns out to be a different key, used to unlock Diamond Island. Olivia explains that it is not the key to the castle, although it does appear to be important.

Mario must now dive down in the center between Club Island, Heart Island, Spade Island, and Full Moon Island to find the Diamond Island dome. There is a keyhole in the center. Mario must insert the key and turn it right side up. This pushes the keyhole aside, allowing them to enter and submerge in the dome.