Red Diamond (Wario World)
From the Super Mario Wiki
The Red Diamond is a treasure from Wario World. Wario needs to gather these treasures from Wooden and Steel Trapdoors in order make the Stone Doohickey guarding the door to the boss of the level disappear. There are eight Red Diamonds in each level, but Wario only needs three to six of them to get rid of the Stone Doohickey.