The jewelry for Rose’s suit is quite simple. An excellent source for Titanic replica jewelry is Karen’s Romance. You can check out her store at www.karensromance.com as well as on eBay. I have several pieces of her collection and I not only wear them with my costumes but with my normal wardrobe also. Notably, this is the last scene in the movie that we see Rose wearing her impressive engagement ring. My replica is very nice costume jewelry but I’m still holding out for a real one. 
All images of the replica jewlery are from Karen's Romance Website. Photos of my pieces didn't do them justice.
A shot of the prop jewelry.
The ring that would have taken Rose "straight to the bottom".