Do you know what sort of ring your fiance wants? Don't buy her something she doesn't love.
At one point I was thinking of getting married. My BF took me to a jewelry store and said in front of the salesman that I could have anything I wanted. I kept asking for something smaller. Salesman couldn't believe it. I just think huge rocks are ostentatious and I don't like them. But, then I may be one of very few women in that.