In Gaston Leroux's The Phantom of the Opera, the ring that Erik had given Christine goes missing:
Suddenly Christine changed color. A mortal pallor overspread her features.
"Oh heavens!" she cried. "Erik! Erik! Have pity on me!"
"Hold your tongue!" said Raoul. "You told me he could hear you!"
But the singer's attitude became more and more inexplicable. She wrung her fingers, repeating, with a distraught air:
"Oh, Heaven! Oh, Heaven!"
"But what is it? What is it?" Raoul implored.
"The ring ... the gold ring he gave me."
"Oh, so Erik gave you that ring!"
"You know he did, Raoul! But what you don't know is that, when he gave it to me, he said, 'I give you back your liberty, Christine, on condition that this ring is always on your finger. As long as you keep it, you will be protected against all danger and Erik will remain your friend. But woe to you if you ever part with it, for Erik will have his revenge!' ... My dear, my dear, the ring is gone! ... Woe to us both!" They both looked for the ring, but could not find it. Christine refused to be pacified.
"It was while I gave you that kiss, up above, under Apollo's lyre," she said. "The ring must have slipped from my finger and dropped into the street! We can never find it. And what misfortunes are in store for us now! Oh, to run away!"
"Let us run away at once," Raoul insisted, once more.
She hesitated. He thought that she was going to say yes... Then her bright pupils became dimmed and she said:
And she left him hurriedly, still wringing and rubbing her fingers, as though she hoped to bring the ring back like that.
The Phantom of the Opera, chapter 13: "A Master-Stroke of the Trap-Door Lover" (translation by Alexander Teixeira de Mattos)
Erik later recovers the ring, and Christine eventually puts it on Erik's finger after he dies.
What's the significance of this scene where Christine loses the ring? Is there any importance to it happening while she's kissing Raoul? Is there some symbolism going on here with the ring?