Blue Nile
Read this portion of the website up, down, and sideways. Then read it again. You must know what you are dealing with, and how much you are willing to spend. Ask as many questions that make sense as you can. Inspect the stone, and learn to identify it. I know my wife's diamond is hers because it has a small occlusion in the 4 o'clock position. Inspect it, ask what the jeweller is looking for, and then look for it yourself.