Observation

The Observation is a location in Luigi's Mansion 3. It is found in the Boilerworks on the second basement floor of The Last Resort, located east of the Reservoir and west of Sewer A. The room's lower level can also be accessed from the Old Reservoir.

The blue gem can be found at the back of the room, where Luigi needs to use the Dark-Light Device to restore a boiler, then flash the Strobulb at the green light on the coffee machine until it malfunctions, revealing a secret compartment.

When Luigi returns with Toad, he can reveal a machine and play music to complete the achievement.