After shopping for some jewelry for my girlfriend I have decided that it is the most uninformed purchasing decision one can make. I am a mechanical engineer and I know a thing or two about materials. Granted, I was far from the head of the class, but can somebody tell me what the heck does 18 karat white gold mean? 75% pure "white gold." White gold isn't an element! It's an alloy. What did they alloy it with? I looked it up on Wikipedia and it can be several things. But jewelry stores don't tell you what it is.
The best thing you can do is know your diamonds. Follow the GIA specs. They are standardized and are how the jeweler purchases the stones. It's the only way to remotely level the playing field between you and the person behind the counter.