The good news is, it's really easy to buy an engagement diamond successfully. The first thing to do is to decide how much money you want to spend. Then go to a jeweler you trust. If you don't have a jeweler you trust, your main choices are mall chain stores, or private independent jewelers. Try both. Go into the store and refuse to be bombed with information. Tell your jeweler how much money you want to spend and have him or her show you what they can offer you for your price. Take a good look at what they show you. Ignore all the mumbo-jumbo. Do NOT look at the stone under any kind of magnification. Find something that looks great to you. You can try negotiating price if you like. Then buy it and give it to your sweetheart. Voila! A successful engagement diamond purchase has been made!
This is a win-win-win situation and is by far the best way to do it. Listening to anything your jeweler has to say (other than dollar amounts) is a serious mistake. They will try to "educate" you. Don't fall for it. Your own eyes are all the education you need.