My advice works if your fiancee is not a big spender on herself:
I proposed to her with a "dummy" ring. The caveat was that I was going to take her to the jeweler so she could shop for her own ring. The idea is that she gets to buy what she really wants while I still get to do the whole ring proposal. Not having the original ring at the time of proposal isn't what she'll remember anyway.
My wife doesn't shop a lot for herself (thank God). She does what I consider a normal amount of shopping so I wasn't too concerned about her going all crazy. I've been told to avoid the mall jewelers too so I went to a local chain which has 4 stores only. When we went, I told her my intention wasn't to give her a budget so much as I wanted her to find a ring that she was ultimately happy with so it was cool with me to go above the quality of stuff she's accustomed to getting herself (basically it's okay to splurge a little). Of course, the $10K rings are nice but to her they were just too gaudy for her taste anyway. We ended up getting a ring that I think was about $2,500 for the engagement and wedding mounted ring set altogether. It was higher than I had anticipated her picking out but she was really happy about it.
Personally, I think a $1K stretch above your budget for a long term investment is worth it. My wife's ring looks really nice (if I'm going to toot my horn here for just a second) and is a constant reminder to me when other women compliment her on her ring which brings a big smile to her face everytime. I've learned that women have similar tastes in quality when it comes to rings and fashion. Totally different than guys. Looking back at what my wife has, no chance I would have figured out my wife's ideal ring. Not a chance in hell. Not sure what your financial situation is, but the dummy ring idea also buys you more time to save up for the engagement/wedding ring. You avoid the whole proposal doomsday clock while giving yourself more time to save money and set it aside.
I know the topic is about engagement ring advice, but the more important decision is making sure she's the right one. The ring is secondary.
