Gold Diamonds are found in two spots. In the first one, Luigi must water a plant three times in the Boneyard (where Spooky resides): once when he first sees it, once after defeating Bogmire, and once after defeating Boolossus to make the now-flower bloom, revealing the diamond and several other treasures. The other can be found if Luigi captures all 50 Boos (not counting King Boo); the last Boo drops the Gold Diamond.