My Dad did that for my Mom, but with a pearl. (Pearls are her favorite stone.) But rather then surprise her he took her window shopping at the mall. Then, when he just "happened" to be in a jewelry department of a store he said, "Why don't you pick out a new ring?" She was floored! She had a lot of fun picking out her new ring, and saves the old one in her jewelry box.
BUT, if it really seems like a bad idea to you, go for a really special piece of jewelry instead. I know women who cherish those gifts just as much.