Elevator Hall (Fitness Center)
From the Super Mario Wiki
The elevator arrow is shaped like a dumbbell. There is a bronze Hammer fountain on each side of the elevator. Luigi can vacuum them to make coins come out of their mouths. To the left is a telephone on a table. If Luigi picks it up, he hears a jingle followed by spooky laughter. To the right are shelves holding trophies behind glass. There is an invisible bag of Bills in the southwest corner Luigi can use the Dark-Light Device to reveal.
To get the yellow gem, both Luigi and Gooigi must stand on the symbol in the middle of the floor, which is actually a pressure plate raising the glass protecting the trophies, allowing them to get the gem.