Color Grading Diamonds

- Color Grading D-To-Z Diamonds At GIA
http://www.diamonds.net/news/NewsItem.aspx?ArticleID=26332

- Color Grading Diamonds At GIA: The First Fifty Years
http://www.diamonds.net/news/NewsItem.aspx?ArticleID=26331

- GIA Master Stones: Only Precision Will Do
http://www.diamonds.net/news/NewsItem.aspx?ArticleID=26330

- Today, GIA uses 'working master sets' of 10 stones to grade the most commonly submitted diamonds, which fall between D and N. Starting with O, the lab doubles up, with one stone per two-grade range, from O-P down to Y-Z. The GIA Laboratory also, however, uses a face-up Z master at the lowest end of that color range to delineate the border between the D-to-Z scale and the start of the fancy-color scale. Because dirt can accumulate on diamonds and affect their appearance, GIA “boils” each master set in sulfuric acid every two to four weeks. The authors reported that some master sets submitted to GIA for review had their color altered by as many as four grades because of dirt that had accumulated on their bruted girdles. To avoid abrasions and chips that can also affect color appearance, GIA lab graders use rubber-tipped tweezers to handle all master stones.

- Now for the human element. Staff members must pass a number of color vision tests and then successfully complete several weeks of training before they are accepted as color graders. Because different individuals can have different color perceptions — a critical concern given the extremely fine borders between color grades — all diamonds receive a minimum of two agreeing opinions before a color grade is assigned. Some stones require more opinions. For example, larger and potentially D-color diamonds are assigned to the most experienced graders. All color graders must take an hour-long break from color grading after every hour of grading to minimize eye fatigue. Round brilliants are viewed table-down on a white tray with master stones in the E to N range — since most of the diamonds graded range from D to M. The grader typically rocks the tray containing the masters and the client stone to eliminate distracting reflections. Fancy shapes at the lower end of the color scale are observed both table-up and table-down, then averaged to reach the overall grade. Potentially fancy-color stones are graded face-up at Z. Rather than attempting to match the diamond to a single master stone, GIA brackets the diamond between two masters, allowing the grader to quickly narrow the potential grade. Once the most appropriate master stone has been determined, the grader observes the diamond on both sides of that master — but never touching it — because the left eye and the right eye may perceive depth of color differently. Fancy shapes are more difficult to grade because they can show different amounts of color depending on viewing angle. GIA has found that the best average color is observed when the stone is viewed face-down with its long axis at a 45-degree angle to the grader, close to the master stone but without any overlap. In this position, it exhibits neither the most intense color nor the washed-out areas.

Great review. Thanks to GIA.