Hi Rubylizard,
I am sorry to hear about your situation with your aunt. I think it's a good idea to confront her, after you have it checked out by your jeweler friend. I think it's possible to communicate with her in a way that will not enrage or offend her, and if she does get enraged or offended and you do have proof the necklace is faulty, then it's her problem to deal with.( I would be very upset if I were you, and offending her would be the least of my worries at that point, but that's just me.)
I was in a similiar situation a long time ago when my grandmother died. She had told me that I would inherit her jewelery collection (alot of it was just costume jewelery, but it was a part of her and to me that is more valuable than anything). I have fond memories of her letting me play in her jewelery box and wear her things, especially a long glittering crystal necklace that I was enchanted with. My evil (and I do mean Evil) cousin convinced my grandmother to move in with her at one point (long story) and I never saw the jewelery, or the other collectibles that she was going to give me (some of which were mine that I had given her) plus a painting that I had painted for my grandmother. I haven't seen or talked to my cousin since, I don't even know where she's at, but I'd rather live with the good memories of my grandmother than deal with my cousin. Sorry for rambling on, I just want you to know that I understand, and I hope things work out as far as getting your grandmother's necklace back in it's original form, the way it was intended.
Love,
Sujin