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The Diamond Key (Japanese: ダイヤキー) is an orange key in Densetsu no Starfy and Densetsu no Starfy 2. It is one of four special keys. The other three are the Heart Key, the Club Key and the Spade Key. They are all based on the four French playing card suits.
In Densetsu no Starfy, the Diamond Key is found in the Sunken Ship. It was originally under the possession of Boctopus. It is one of four keys required to open Sunken Ship's prison cell and rescue Dolchina and Dolshi.
The Diamond Key is found through the bottom-right door of the main room (with the prison cell). Starfy must solve four rooms of rotating block puzzles to reveal the Diamond Key behind the last room's four 'square' blocks.
In Densetsu no Starfy 2, the Diamond Key is found in Ogura Castle (Stage 11-4). It is one of four keys required to unlock the blue cage Moe was trapped in after he fell for an illusion of Ruby created by Ogura. It is found through the bottom-right door. Starfy must navigate a room with Sukebon. Like the room that contains the Club Key, Starfy must pass a room with several Stone Pillars. In this room is one of the two notes.