@Seracila nice one....yes I agree that the right choice would be to say that she is more important than the pair necklace
second of all, Kabakura needs to
show that he cares , he shouldn't say it's not a big deal/it's just something cheap after seeing her basically turning the room upside down...at least say "I'll tell you if I find it"
third Kabakura should try to cheer her up, tell her that he just wants her to be happy...maybe say "tbh I'm more worried to see you so down rn...losing a necklace doesn't mean our cherished memories are lost, so don't be so sad..ok?"
and maybe go for the kill by saying "so lets make more memories with another thing that matches instead" and then BAM! take out the engagement ring lol jk
...and yes I read too much romance manga