If I am not mistaken, GIA diamond certifications are the consensus
report of five graduate gemologists. If you were to send the same
diamond back a second time, there is small possibility that you might
get one grade change in color or clarity, but nothing obscenely
different. If you were to use a one-man lab or appraiser, you would be
trusting the judgment of a single individual versus a team of five. On
the one hand, an expert individual might give more consistent,
non-compromised reports. On the other hand, it would seem pretty
unlikely that a group of five gemologists would miss anything major.
Donna Shimazu