So… after careful evaluation I’m pretty positive Elio puts Oliver’s necklace in his mouth during the midnight love scene. Oliver’s necklace falls off as he’s getting up after Elio had tugged at his neck whilst laying down.
After this… and then when he gets up
You can see the necklace on Elio’s belly there.
I’m not sure if this was intended , but it’s most likely one of their improvised scenes. The necklace probably fell off by mistake and before they cut to the scene of them lying together Oliver had put the necklace back on bts.
Then let me say one more thing. It’ll clear the air. I may have come close, but I never had what you two have. Something always held me back or stood in the way. How you live your life is your business, just remember, our hearts and our bodies are given to us only once. And before you know it, your heart is worn out, and, as for your body, there comes a point when no one looks at it, much less wants to come near it. Right now, there’s sorrow, pain. Don’t kill it and with it the joy you’ve felt.