The item Gold Diamond does exist. You may find out more information about it by clicking here.
Luigi obtains a Gold Diamond

Luigi obtains a Gold Diamond.

Gold Diamonds are the most valuable treasures in Luigi's Mansion, Luigi's Mansion 2, and Luigi's Mansion 3: The Room of DOOM!, having an amount of 20,000,000G each. In all three games, there are two Gold Diamonds that are found in each game, which will total 40,000,000G when they are both collected. In all games, there are hidden plants that produce a Gold Diamond and other treasures when they are watered three times without forgetting a single time. The second Gold Diamond can be obtained by sucking up all 50 Boos in each game. The last Boo that gets sucked up will produce the second and final Gold Diamond. To get an A rank mansion, Luigi can most likely do so by getting both Gold Diamonds.

Interestingly, Gold Diamonds are larger than Silver Diamonds and Bronze Diamonds. This can be noticed if one looks close enough.