The Professional Gem Science Laboratory (PGS) is also a dependable lab, but doesn’t have a significant market share.
Moreover, GCAL is also a very refutable lab, but have a small market share.
The only factor you have to lookout for is the cut in GIA reports.
In round cuts, they offer a cut grade, but they do not in other shapes.
We strongly recommend GIA as the top lap and also consider AGS, HRD and GCAL as reliable labs.
The International Gemological Institute (IGI) is also improving how they grade diamonds.Overall, GIA is still considered as the standard-bearer in the industry when it comes to the education and grading of diamonds.Here are our notes on IGI vs GIA as well in case you are interested.The only reason they would grade it by other labs is if they think they can get better results from the other labs as opposed to GIA.What we have seen is that GIA is extremely strict/professional in grading diamonds.Therefore, they have a huge interest to sell you a loose diamond with no certification at all or one graded by a lab that is lenient/soft with their grading criteria.No diamond dealer would avoid sending their stone to GIA to grade if they are confident about the quality of the diamond.In short, diamond certification is extremely important.Do not buy a loose diamond that is not graded by a reputable lab.Sometimes you might get conflicting opinions because gemologists might tailor their opinion to the lab that graded their diamonds in order to make a sale.These opinions are obviously biased and you should vary of such opinions while shopping for a diamond.