The value and price of a diamond is based on four factors: clarity, color, carat weight, and cut, also popularly known as the 4 C's of diamond grading. Of the four C's, the diamond's cut is largely responsible for enhancing its visual appearance and sparkle.
With reference to diamonds, cut has two meanings. The first meaning is its shape, which could be oval, round, etc, and the second is execution of a specific type of cut within the shape of the diamond. Cut grading is the rating which later determines its cost and quality.
When a diamond has the best cut, the incident light falling on it passes through the crown, table, and finally the pavilion, where it gets reflected from one side to the other before escaping through the diamond's table again. Such a cut is also known as the ideal cut and this phenomenon of light passing through the diamond is called "light return".
The diamond's brightness, dispersion and brilliance is enhanced or depreciated based on the movement of the light within it. A bad cut or a slight amount of asymmetry severely hampers the quality of the diamond as it causes the light to escape out of the diamond prematurely. This phenomenon is also called "light leakage".
Till only a few years back, evaluating the quality of a cut was a very intricate task. Very few people, having a trained eye were able to determine the quality of the cut. However, with the advent of new grading systems of AGS (American Gem Society) and GIA (Gemological Institute of America), grading diamonds has become a far easier activity.
GIA (Gemological Institute of America)
According to this system, the diamond cut grade depends on its design, face-up appearance, and also the craftsmanship that has gone into it. The fire and brilliance of the diamond is evaluated by considering all these factors together. Based on these parameters, the GIA grade system rates diamonds as "Excellent", "Very Good","Good" "Fair", and "Poor".
GIA's diamond cut grading system uses 70,000 individual observations, 38.5 million proportion sets and a computer based predictive model to perform the cut assessment. The system came into effect in January 2006 and most of the diamonds graded before will not be entitled to the laboratory-assigned cut grade.
AGS (American Gem Society)
AGS's grading system considers 11 combinations of three parameters namely proportion, light performance, and finishing properties to evaluate the fire and brilliance of the diamond under consideration.
Based on these evaluations, the grade ratings of "Ideal", "Excellent", "Very Good", "Good", "Fair" and "Poor" are assigned. A diamond that meets AGS's perfect standards of symmetry, polish and proportion for cut grade is termed as "Ideal", and it is also popularly known as a "Triple-Zero" grade.
Difference between AGS and GIA
GIA does not provide an "Ideal" cut grade rating, and an AGS "Ideal" cut grade is considered equivalent to the "Excellent" diamond cut grade of GIA. Also, GIA provides a symmetry demerit or "non-standard brillianteering" as it is referred to by GIA sometimes, to help manufactures to improve the standardized Tolkowsky cuts.
AGS uses a more accurate combination of the proportional facet ratios to determine the quantity of light return in the diamond. It also takes into account the ray tracing metrics for this purpose. In case of GIA, however, the cut grading system is based on averages which are rounded off to determine the light return phenomenon.
So, before visiting the jewelers to buy some diamond jewelry, don't forget to read the GIA diamond dossier and AGS report, so that you can evaluate the cut gradings correctly and get the best value for money.