it sounds to me like your grandma also knew if your mother got it you never would - so she sidestepped your mother to make sure it carried on down the family.
With that in mind, I would tell your mother that it is yours, not hers, and that had grandma wanted her to have it then she would have given it to her not you.
if she argues further, tell her to talk to grandma about it.
Respect your elders. And since grandma is older, she comes first. It's true, though. Grandma gave them to you for a reason. If she wanted your mother to have them, she would have given them to her herself.
It was a gift to you. Your mom should deal with grandma if she wants it. You have no obligation to pass your gift on to someone else. If your mom's grandma had wanted her to have it, she would have given it to her.
Whether alive, or in a will, your gran has the right to dispose of her things as she sees fit. MANY times this has ticked off greedy relatives, so don't think you're the first. Memaw (great gran) has some pearls... they're poda go to the daughter... so they'd go to Gran (already passed) then to my mom, and Mems says then to me, as eldest, as opposed to my sister... my mom expects this to cause a big tadoo in the future, Mems hasn't passed yet, but it's close hence this conversation. Personally, I'd rather have the irons from her kitchen that were her mothers or even the Bud Clock I know hung in my Gran's Cafe for years ... I don't wear a lot of jewelry... but I'd respect her wishes and pass them to my daughter... my sis on the other hand would have them sold before a week was up... no doubt at all sad to say... but I didn't say that... I was trying to be nice... but one comment still slipped out... I said to mom "If Mems gave me a paper bag and Ash didn't get one then she'd be upset"... sad to say that mom agreed... so there ya go.
Sounds to me like your Granny is as sharp as my Mems. She knows her children, grands, etc. knows which will do what with what. Knows which will treasure her treasures and which will just see dollar signs, and thus she's taken the precautions that she has.
If my sister showed up demanding I give the gift my Mems left me so she could horde or sell it I'd tell her to kiss my toe... but that's me... I know just where I got my backbone from and I'm not afraid to use it if the cause is just. Honoring a dying woman's wish is a just cause IMHO... selling out that wish for new shoes or drugs (MY sis, NOT your situation at all) is just a load of crap. Again, just my opinion, but one that I've had no choice but to think about since my mom dropped that bit of news on me. Actually it's 10 to 1 that she'll just give them to my sister anyways... she ALWAYS does... keeps enabling... *sigh* but that's way far off topic so I'll stop now.