The Maestro's Ring is a ring worn by Maestro Delphine Eraclea.
The ring contains a piece of Guild technology which kills anyone who touches it other than Delphine by causing their body to disintegrate into a salt-like powder if he had done alot of bad things in his life like the auction's manager, or into tiny pieces of sparkling glass if that person was good and had a good heart, like Lucciola. Delphine uses the ring as a means of execution, granting it to people who displease her as a "gift" and then reclaiming it after they have died.
Two characters are killed on-screen by the ring: Giancarlo and Lucciola. Giancarlo was unaware of the ring's properties and truly believed it to be a gift; Lucciola knew what the ring did but willingly accepted his death in exchange for Delphine granting Dio Eraclea his freedom.