That would be considered rude. Even though she may not wear it; she may still value the thoughtful gesture that you demonstrated when you presented her with the necklace. Use the money the buy yourself a new necklace. Because of the Valentine's season; jeweler a are offering many deals.
True, it is never proper etiquette to ask for something back that was given as a gift; however, if she isn't wearing it you can raise the subject by telling her that you lost yours. Rather than offering to buy hers, be honest and tell her you miss wearing yours. Ask to "borrow" hers if she's not wearing it and promise to give it right back no matter what, no matter when she asks.
Another creative option I've seen happen with friends and even siblings is 'sharing' something. One example I know of is two sisters who hoped to inherit the same bookends. Their eccentric aunt knew it and left one bookend to each sister with the suggestion that they put them together and every year at Christmas, give the set to whichever sister didn't have them. So there's no argument, no questions, this is what they do - and they have fun doing it. It means that every other Christmas one sister gets out of having to the other sister a Christmas present
In any event, asking for it back or asking to buy it back isn't the solution, but an open dialogue with honesty is. I have a feeling she'll be happy to let you 'borrow' it or set up a 'share' .
The day will come when she finds the same attachment to this one remaining necklace as you do and by then, when it's hers all the time, you could have enough put away to find another one, just for you.