Diamond Steppe Island looks like a diamond from above. The island is also terraced—rising sharply in elevation like a set of stairs in some areas. You need the Hookshot to traverse this island.
Orange Rupee (via Big Octo)
This sector has an eight-eyed Big Octo. Sail toward a flock of circling seagulls to find it. Use the Boomerang to target up to five of its large, yellow eyes at a time. When all eight eyes turn blue, the defeated Big Octo leaves behind a ring of light. Use the Grappling Hook to haul up a chest containing an Orange Rupee.
Piece of Heart #21 (via Treasure Chart #23)
Find and open Treasure Chart #23 (found at Windfall Island [D2]) to pull a chest from the Great Sea near Diamond Steppe Island. The chest contains Piece of Heart #21.
Blue ChuChu
Defeat the Blue ChuChu on the third level of this island for a Blue Chu Jelly.
Ghost Ship Chart (via Secret Cave)
To find the Ghost Ship Chart, drop into the hole on the top level to enter the Secret Cave. You must travel through several Warp Pots to reach a chest that contains the Ghost Ship Chart (an optional chest contains a Joy Pendant). If you have difficulty with the Warp Pot puzzle, please refer to the walkthrough (p. 236).