Satoshi must use it on the evil spirit outside of the room in the hallway. The crystal will shatter, sealing the spirit and freeing the other party. If the crystal hasn't shattered, it can be used during the battle against the Vengeful Spirit. It will restore 10 MP to all party members and has infinite uses.
CORPSE-PARTY ZERO Details
The crystal seems to emit a strange light.
Use it to free Shiho Hasegawa in the girls' lavatory.
The item is a combination of 3 crystal fragments found throughout the school. After you collect them you can give them to a ghost in the stretching hallway and it will combine them into the crystal cluster, which can be used to free Shiho from the mirror in the lavatories.