With all due respect to other people above, do NOT go to Sam's Club or Costco to buy an engagement ring. Do you really want the rest of your married life to start at a wholesale store? I understand that it's convenient, but come on. Here are the unsaid rules about engagement rings.

1. Rule#1: Stay Classy
   
   1. Your engagement is hopefully the last time you make a huge commitment like that. Do you really want to look back 20 years from now and think: "Oh yeah, I just went out and bought this thing that is a symbol of our relationship. It was on sale at Sam's club. I also got some cheap laundry detergent and it was like two birds with one stone!". That's such a थर्ड क्लास move. You want your marriage symbol to be something you put some effort in. Something that you spent some quality time researching, and something that is "perfect" whatever your definition is.
   2. There is a reason I put this rule in huge bold letters, because everything I will say revolves around this. I will often refer back and say "See Rule #1"
   3. (Special note: I am aware that an engagement ring may not mean everything and how people can be stuck in materialism. But if you are going to buy an engagement ring, you must do it in a classy manner, or not do it at all. Half-assing causes a lot of problems. If you want to say "Screw this, our love is beyond that" then don't get her an engagement ring at all.)
2. Rule#2: Don't get an engagement ring online
   1. I understand that it is very convenient to get an engagement ring online. However, there are a lot of problems associated with that. This idea is mostly preference, but I really like to see and feel what I'm buying before I actually make a purchase. You can get better deals online, but an engagement ring is not like a television or a computer that you can buy from Amazon.com. A diamond, even though it has its' own specificiations (which I will get into below), is still unique piece of rock. Each diamond is unique and you want to make sure that it's the diamond you want to see in your future wife's finger for the rest of your life. I used to work at a nursing home and a lady had been married for 75 years. Her husband passed away, but she still wore her old diamond ring.
   2. Once again, refer back to Rule#1. You can buy a TV online, or a computer online, but a diamond ring should have a little more effort than clicking on some links on your computer.
   3. I bought mine from a local jewlery store. I would avoid chain stores as well. Like gwachaquarion mentioned over there--they try to lure you with their gift certificates, and don't end up giving you a good deal. You are better off going to a local jewler who has his own business. It'll help the local economy, plus you'll have a good jewler to refer to when you have to make other purchases. The jewler I use also cleans the diamond ring for us every month, which is fantastic.
3. Rule#3: The 6 C's of diamonds: you may have seen a variation of this as "4 C's of diamonds", but I added the last two because I feel like these are important as well. You can learn more about these C's in full detail in this web site.
   1. Cut
      1. A cut refers to the reflective qualities of the diamond. Don't confuse it with the shape (round, pear, emrald). This is the most important quality of a diamond.
      2. A good cut gives a diamond an allure. It will also make a diamond look brilliant. This is the "Sparkle" that you get.
      3. Cut also is the cause of the reflection that you get from the diamond. A poor cut will mean the diamond will look more dull than a better cut diamond
   2. Clarity
      1. More clear = more brillIance. As simple as that.
   3. Color
      1. Less color = most refraction = more sparkle
      2. You would want it to be as colorless as possible
   4. Carat Weight
      1. The higher the carat, the larger weight the diamond is
      2. It also means more $$, and the price increase is dramatic. A 1.2 carat diamond could cost you over $4000 versus a 1.1 carat that could cost you $3000. This depends on other factors as well. I would at least go for 1.0 carat.
   5. Certificates
      1. This has been mentioned by folks above my post
      2. Just be sure that your diamond is genuine
   6. Cost
      1. This is where it usually comes down to. Usually financing is available thorugh jewelry stores. It is better to pay as much of the principal as possible. You don't want to buy an engagement ring in complete debt. But if the right ring for your fiancee is available for a slightly higher price than your budget, and financing is available, it's better to go for it. You're only going to buy it once!
      2. Here's my take on it: you just have to go for it. I bought an engagement ring for slightly above my budget, but the difference was of $10 a month. By that point, the $10 difference wasn't going to be a big deal. The nice thing was that the store had a very good deal for me too. If I had bought the ring at a retail stores, you can't really get the price knocked down. I was able to butter  up the store and get it down by almost 25%, which was fantastic.
4. Rule#4: It's about the thought, not just the gift
   1. Starting a marriage is a huge step, and you need to be sure that you want to spend the rest of your life with the person you are proposing to. You need to be sure that the person isn't just marrying you because she got a shiny object.
   2. Having a strong marriage starts with love and commitment. It also comes with sacrifices that both of you will have to make for each other.
   3. I would suggest getting pre-marital counseling. Your neighborhood churches do that very well, and so do clinical psychologists. I think talking about possible issues that may come with marriage must be discussed, otherwise it will cause lots of problems. Communication is the key! You can get more information here about that.

Hope all this helps! Best of luck in your new life.