Every individual diamond is unique.
Every individual person is unique.
Diamonds are graded by individuals.
So it could easily be assumed that the same diamond, if graded by two different people, would receive two differing grades.
This can be the case, though some more widely recognized laboratories have a greater degree of consistency across the board than smaller laboratories.
In June 2013, the Rapaport Diamond Trading Network (RapNet) got back the results from 10 diamonds graded by half a dozen different brands of diamond grading laboratories. The degree of variance between grading laboratories was not always extreme though one thing was clear: some laboratories are stricter. The GIA labs ranked as the ones demanding the highest quality for a diamond to receive a high grade. An article on RapNet's analysis was published here: http://www.diamonds.net/news/NewsItem.aspx?ArticleID=43417
There was a surprise, though, in that IGI labs were almost on par with GIA and ranked higher than labs in the EGL USA network. However HRD, fell below EGL USA, followed by EGL Hong Kong and EGL Israel.
As the name on a diamond's grading certificate will affect the pricing, RapNet calculated how pricing had factored into the mix based on the reputation of grading laboratories. The result was that IGI diamonds are suffering from an unwarranted besmirched image. According to this report, two diamonds with an equal grade from GIA and IGI should be seen as fairly equal. Yet the IGI diamond will invariably be listed with a lower price than the GIA graded diamond with an identical grade.
The RapNet analysis concluded that it is inadvisable for a diamond shopper to simply get sold on a particular brand of certificate, but to buy the diamond, not the certificate. This would mean establishing a good line of communication with the jeweler and getting a clear understanding of the qualities of the stone.
My conclusion is that looking at diamonds with an IGI certificate could turn up some very nice bargains.