Gold Diamond

The Gold Diamond is the most valuable treasure in Luigi's Mansion, as each of them is worth 20,000,000G(s) each, equal to 4,000 gold coins or red diamonds. There are a total of only two Gold Diamonds in the entire game (totaling 40,000,000G(s) if you get both of them. The first of them is rewarded by watering the plant in the Boneyard three specific times (where the portrait ghost  Spooky resides): the first time when he sees it, the first time after defeating Bogmire, and the first time after defeating Boolossus to make the now-flower bloom and open up, revealing a lot of Moneys and the Gold Diamond. The second of them can obtained after Luigi captures all the 50 Boos (note that the game doesn't count King Boo as a Boo); the last Boo will place the treasure/diamond right in front of him. If Luigi wants a beautiful house to live in (an A-Rank), that means he should obtain both of the Gold Diamonds.

Trivia

 * If the player looks closely, he/she can see that the Gold Diamonds are larger than the other diamonds, The Silver Diamond and  the Red Diamond. Which makes them the largest treasures in the game.
 * Though they are the most valuable treasures of Luigi's Mansion, in the PAL Hidden Mansion, collecting all thirty Silver Diamonds results in 60,000,000G(s), as opposed to two Gold Diamonds, which is only 40,000,000G(s).
 * There is a glitch that when the player captures the 50th Boo, sometimes, Luigi will both capture the Boo and collect the Gold Diamond at the same time, leaving him talking to E. Gadd while actually holding the diamond. This can be seen here.
 * Note that if Luigi loses all of his HP, or if the player exits the game after collecting the second Gold Diamond from capturing the 50th Boo right after the player saves, it will disappear from Luigi's inventory and from the room the green-hatted plumber collected it from and it will never come back or recover, making it impossible to collect for the rest of the game. Because of this, it's more recommended to not save after capturing the 50th Boo, even if it means having to defeat Van Gore over again.
 * The Gold Diamond does not appear in Luigi's Mansion 2, same with the Silver Diamond.