You have already gotten great advise by checking with GIA. But in addition you should buy from a reputable vendor and someone with a GOOD RETURN policey should everything not check out! I actaully don't mind buying diamonds that aren't GIA certified as long as there is a good return policey so I can have everything checked out.
Challenge is that people are using copies of real GIA certificates and passing them off for other diamonds. Best to check the basics and then verify with an appraisers. I would check on my own weight, measurements and inclusion plots.