So you want to buy an engagement ring, but you're not sure where to go. The first option to consider is the big diamond houses: Cartier, Tiffany's, Harry Winston.
If you're concerned about brand and assurance quality, you can't go wrong with a major retailer. However, you are going to pay the price for a brand name.
Another great option is going to someone in the wholesale trade. You can ask a friend, or a friend's friend, or a friend's cousin Vinnie who works in the diamond business, someone that can help you in that area.
Or you can find a private jeweler like myself. I bridge the gap between retail and wholesale, so I can help you find a loose stone and create the perfect custom setting.
You can buy online. I think it's a little dangerous to buy a stone say unseen, but a lot of these great online sites have certified stones so as long as the four c's all add up, you're in pretty good shape.
The most important thing is to pick one jeweler, feel comfortable, because you want to have a relationship with this person for the lifetime of your ring.