first off.. if you tried the dress on and it made you think its the one.. then it prob. is. dont pick a dress just to go with your mom's necklace. and your right, i think that would be too much on the neckline. your mom will be hurt, but i would tell her. and try and get other's opinions in front of her, and hopefully they will agree with you. maybe you could find a way to wear it some other way( like a bracelet, or under the dress as something borrowed?) to make her feel better. but hey, if you dont want to do that either, dont! its your wedding..you want to wear your own dress and jewelry.