Accurate Grading in a Mouting
The only way you can assure that a diamond is accurately graded --insofar as the grader's ability-- is to view it loose. If it's set in a mounting of any kind, the best you can hope for is an educated guess. A bezel- set diamond is the most difficult with the diamond's girdle completely covered. Prong-set stones will also be challenging, but with much less hindrance.