Luigi collects a Red Diamond.
Bronze Diamonds are diamonds with low value that appear in Luigi's Mansion 2 and Luigi's Mansion 3: The Room of DOOM!. Surprisingly, they are worth 5,000G, which is the same amount as a coin. In both games, there are two Bronze Diamonds that are hidden in the mansion, which will total 10,000G when both are collected. The first Bronze Diamond is found by watering a plant, and the second Bronze Diamonds are found by collecting King Boo's crown, which is automatically given to Luigi when he sucks up King Boo whole.
Although Bronze Diamonds did not appear in Luigi's Mansion, similar treasures known as Red Diamonds appear. There are two. They have the same value as Bronze Diamonds, and their ways of being found are the same as the ways of finding Bronze Diamonds in Luigi's Mansion 2 and Luigi's Mansion 3: The Room of DOOM!. It was stated by Fantendo and Nintendo that Bronze Diamonds replaced Red Diamonds.