Red Diamonds are surprisingly unvaluable and obscure treasures in Luigi's Mansion. Unlike the Blue and Gold Diamonds, each Red Diamond is only worth 5000G, the same amount as a Gold Coin. There are only two, and one of them is King Boo's crown, automatically given to Luigi when he sucks up King Boo after his epic battle on the roof. The other is found by watering a plant on the Balcony.
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The Red Diamond from Wario World.
Another, different Red Diamond appeared in Wario World. Wario needed 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 were eight Red Diamonds in each level, but Wario only needed three to six of them to get rid of the Stone Doohickey.