Diamond Key
From Starfy Wiki
The Diamond Key (Japanese: ダイヤキー) is an orange key in Densetsu no Starfy. It's 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 other keys are the Heart Key, the Club Key and the Spade Key. They are all based on the four French playing card suits.
It's 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.