In the musical "The Phantom of the Opera," what significant item does Christine leave for the Phantom at the end of the show?

In Act Two of The Phantom of the Opera, Christine stars as the leading role on the opening night of Don Juan Triumphant. Although she is tasked with singing with Ubaldo Piangi, she soon discovers while on stage that she is instead singing with the Phantom himself. The Phantom then kidnaps Christine and brings her to his lair, where he attempts to force her to marry him in order to save the captured Raoul's life. Christine kisses the Phantom, which changes his mind and causes him to agree to release the lovers.
Before fleeing the lair with Raoul, Christine returns the ring that the Phantom had given to her, and he professes his love. This is a powerful moment in the musical, as the Phantom is experiencing something he had not experienced before: true feelings for another human being and a willingness to show compassion.