When buying a diamond, a certificate is your best friend. It’s your guarantee from a third-party that you’re getting what you’re paying for. However, you should be wary of who is issuing the certificate. The leading names and authorities in the industry are: GIA (Gemological Institute of America), AGS (American Gemological Society) and EGL (European Gemological Laboritories). These are all trustworthy, but you should be skeptical of anyone else.
Here is an NBC Dateline report on inflated grading from other labs.