The whole precious stone market is a scam. Every stone is different and GIA ratings are still someone's opinion (even if they aren't lying about the quality ratings). Unless you take the time to learn how to appraise a stone yourself, you are likely to get screwed. It takes a trained eye, a little knowledge and the right tools to evaluate a diamond, so Joe Consumer wouldn't know quality from crap when it's sitting right under his nose. Since people are conditioned to shop by GSA ratings, jewellers can say the quality is whatever they want to to drive up the price of the stone. And, trust me, every jeweller is looking to maximize the return on their investment. Wouldn't you?
Always get an unbiased appraisal from somewhere else before buying, especially if you are clueless about appraisal. A couple of buying tips; you are guaranteed to get screwed if you buy from a store in a mall; always look at a diamond away from the bright halogen lights (best is to look at it under flourescent lights); ask the jeweller if you can borrow his loop, or, even better, his microscope. Even if you don't know what the hell you are looking for, he still might get worried that you do. Try to find a used diamond from and estate sale. You may be able to get it a bit cheaper and, it's not like they break or go bad.
BTW, this can work to your advantage. If your chick wants a 3K stone, find one with a decent color, but with inclusions and a poor cut. You can pick that up really cheap. Still looks like a 3K diamond to her, but it's less brilliant and has no resale value. She probably won't ever know the difference. I guess it's true what chicks say...size does matter.