How dumb can he be?
Like I get it if he don't understand at first because the pendant doesn't mean that as much to him but like even at the end of the chapter he still don't understand the pendant means more to her than a simple pendant. At this point he must be really dump but also he mustn't try very hard to understand her.
It means a lot to him too though, otherwise he wouldn't worn it all the time and keep it for sooo long! But I think he cares so much more about her being sooo sad than an object he bought in the past X'DD
I agreewith @keks503 as well.
Also, knowing his is the one that bought it, it's easier to understand that you would cheerish a present you get from your loved one more, than what you buy to give as present to your loved one.
i hope he hurry realize that the necklace that he give her are the first present that she treasure so much just like how she treasure him even though she show it in her way to show how loved he is
Even if Kabakura understand, what is he supposed to do though?
He can't make lost item magically appeared and maybe he is not the type of person that would say "you are more important to me than the necklace" right away.