King Boo's Crown
From the Super Mario Wiki
King Boo's Crown is the final piece of jewelry Luigi collects before he saves Mario in Luigi's Mansion. In Luigi's Mansion, the crown holds a red diamond at the top. Officially, King Boo's crown also counts as a Red Diamond, making it worth the same amount as a single coin. King Boo's Crown is the only unit of money in the game that must be collected, so it is possible to get only 5,000G the entire game.
The crown has changed in the later Mario games it has appeared in. It is nowadays shown as a golden crown encrusted with jewels similar to Princess Peach's crown, with four red ones encrusted around the crown and one blue one in the center. In Luigi's Mansion: Dark Moon, however, King Boo appears to sport the crown's initial appearance, except the gem, which is now a purple, glowing, diamond-shaped stone that resembles an amethyst, matching King Boo's color theme. This gem gives him enough power to shatter the Dark Moon.
The crown returns in Luigi's Mansion 3, alongside its owner. It largely has the same appearance as in Dark Moon, although it is given a bit more detail. After defeating King Boo, Luigi manages to gain the gem. However, after the hotel collapsed and E. Gadd's containment field was breached, the gem vanished which also caused the volatile ghosts to return to their senses.