From the Super Mario Wiki
The Ceramics Studio (Japanese: 壺置き場 Vase Storehouse) is one of the many rooms in Luigi's Mansion. It is cleared in Area Four. After Luigi enters the Ceramics Studio, he fights Jarvis. After Luigi defeats Jarvis, he receives a green treasure chest with many treasures, the most valuable being a Blue Diamond.
The Ceramics Studio contains nine jars and vases, many of which contain treasure and one even contains a Red Ruby. There are four on the north and south walls. The jar near the center of the room is frozen and contains Ice Elemental Ghosts. Some crates and chairs are stacked behind the crafts. In the top right corner of the room is the chimney for the fireplace in the Study, blocked by wooden boards.