A gemological certification is an official quality report on a diamond. The official certificate includes information about the exact measurements, proportions, color grade, cut grade, clarity grade and carat weight of the diamond. When comparing gemological labs, GIA and AGS have the highest ratings worldwide in the diamond industry:
GIA: The Gemological Insititute of America
AGS: The American Gem Society
Labs other than GIA and AGS are of very little value when comparing certificates due to evaluation errors. This is very important when comparing diamond before purchasing your own.
*note: when comparing lab certificates, an official GIA/AGS documented certificate is of much higher value versus the mere opinion of a GIA or AGS trained professional or appraiser.