This is an old article but it was so interesting I thought it worth regurgitating. Back when the internet was new, The Wall Street Journal put online diamond dealers to the test. But as the Gemstone Forecaster pointed out, there were some problems with their methodology:

The Gemstone Forecaster has one important problem with the methodology of this report. Of course, we do not know Peter A. Natale, the Manhattan jeweler utilized by the WSJ to verify the purchases. What we find unusual is that here is a jeweler who believed he was expert enough to disagree with the GIA’s grading of Ashford.com’s one carat, G color, VS2, but refused to grade Blue Nile’s one carat, G, VS2 diamond because it had an IGI grading report. It is even stranger to us because he probably had all the stones together, including two other GIA graded G-VS2’s. We find this extremely suspect and it does not make sense.