How do i locate GI REPORT on emerald cut diamond purchased@neiman marcus approximately 1965-1970,?

My birth father, now deceased purchased my mother's engagement ring at neiman Marcus, either in dallas texas or new york, aprroximate year 1965-1969. it is a 5 caret flawless diamond, emerald cut, with baugettes, set in platinum. i have made a grave error and took it to have appraised for insurance purposes. when i went to pick up the ring i was told that it is of far less clarity and color than would be substaniated in the original GI report. Neiman marcus would not even sell the quality of diamon below a "F" in color and this was a "D: I was FOOLISH and stupid for leaving such an expensive, treasured family heirloom with an unkniwm jewler, who obviously has switched the diamonds on me. he is still attemting to return a five caret diamon but of I color and week clarity. i did not accept the diamond and filed a police report (this is the only time it has ever been off my mother's hand), but i desperately need to find a way to prove proof of purchase from neiman marcus or obtain the g