GIA’s Gem Trade Laboratory (GTL), has collected sets of diamonds for the purpose of defining the color grading system. These sets are complete including the very highest grade down to the lowest and beyond that to fancy colors. When a diamond is sent into their lab for a full report or color origin report, it is based on these master stones, which are compared to the diamond in question to establish its color grade. A master stone is the absolute lightest color in its assigned range. Remember these color grades are in narrow ranges, not on a specific point. For example if the diamond in question is ever so slightly better than the master “G” we know it is an “F” color.