I buy a lot of schlocky fashion jewelry on QVC. In case my financial situation improves, I've visited Tiffany's, Cartier, Van Clef & Arpels, Harry Winston and Buccellati recently. One thing was clear. Yellow gold is in among the high end jewelry buffs.
I was told it is much better to have a few significant pieces of good quality than a bunch of junk. My window shopping expedition was instructive. Once I saw the quality at the fine stores, I did not want anything less so I am now satisfied with fashion jewelry.
I love Art Deco and Art Nouveau pieces.
It may not be romantic but consider a special card announcing your intention to give her a piece of jewelry. She should pick out what she wants. Maybe you know her type and what she likes. It is so personal. I would go for quality over the big end stuff. Pearls without lustre that could never be legally considered pearls are now ubiquitous. I prefer pearls to diamonds.
An Hermes scarf ($400) also makes a great gift.
Miriam Haskell and Kenneth Jay Lane do interesting fashion pieces.
Unless you have at least a one carat solitare from Tiffany's, Cartier's, etc., it may be a crummy invesment.
Antique compacts, cameos, stuff with character are available through stores that sell vintage. Cartier's and Tiffany's were selling their vintage pieces when I visited them. The old stuff was more impressive than the new. Bottom line is that it is very personal.
I'd like a smaller, token piece with a charitable donation to a cause I like.