In the diamond trade, «Cut» consists, conventionally, of two separate grading parameters: «Polish» (usually mentioned first), or, the actual finishing of the different facets' very surface, indeed, their polishing, and, «Symmetry». In order to maximize brilliance, the diamond cutter must place each of the diamond's facets, which act as light-dispersing mirrors, in exact geometric relation to one another. The symmetry of the placement of the facets, the integrity of each facet's borders, its alignment with all other bordering facets, the issue of whether the table plain is parallel to the girdle plain or not, whether or not and the extent to which the table or Culet are off-center, and perfect closure of all corners of each facet are contributors to the diamond's overall «Symmetry» grade. Since «Cut» indicates the combination of «Polish» and «Symmetry», it is not uncommon to see expressions such as «Very Good/Good» etc. indicating a «Very Good Polish» and «Good Symmetry», etc.
Some diamond grading reports now also include grades referring to the diamond's overall «Finish», «Proportions» and even «Light Return» performance. The latter, especially, is based on a 3D diamond modelling and ray-tracing within it, based on calculations in powerful computers. The «Light Return», in particular, is an excellent indicator of the diamond's performance, namely, its reflection of white light rays from within it, the closest one gets to the traditional definition of «Brilliance».
Please notice that the GIA's highest Polish and Symmetry grade is «Excellent», not «Ideal». «Ideal» is used only by several other grading laboratories.
Sometimes, in reference to the diamond's shape, as in, e.g., «Radiant Cut» the word «Cut» is used, but in a sense that is unrelated to its use in «Cut grading». Some refer also to the diamond proportions as «Cut». Read about «Cut» when proportions are meant under Cut Performance.
The shining of a diamond, or, its performance, is analyzed in terms of its brilliance (all white light returned to the eye from inside the diamond), dispersion of light, (its fire and colorful sparks), and its scintillation, (its surface reflection of light by each facet).
While the scintillation is a function, mainly, of its polish quality, the brilliance (white light return) and fire (colorful spark shooting) of a diamond are determined by its proportions, or, the relative size of its different measurements when compared to each other, and the various angles between crucial parts of its geometrical shape, commonly referred to as the diamond cut's Proportions.
While recent computer models have generated some curious, untraditional greatly performing diamond cutting proportions, the bench mark proportions known to generate stunningly performing diamonds are those known in the trade since 1919 as «Ideal Proportions» (recent computer models validated those but added some others still under study). With the years, and the trade's growing experience, the original set of Ideal Proportions as calculated in 1919 was extended beyond its original scope, and, based on the closeness to the original Ideal Proportions, a diamond's performance may be predicted.
The significant of the various angles created in the cut diamond cannot be underestimated: unless within a strict set of parameters light that falls on the diamond's top - its «Crown»; - will not be fully returned through the Crown again, from where we normally view it. Consider the following diagrams:
As you can see in the above diagram, figures (b) and (c), both too shallow and too deep a diamond cut, as determined by the various groups of inclinational angles of the various facets, allow the leakage of some of the light falling on the diamond's top - its Crown. Proper proportions will also disperse the Light Falling on the diamond's Crown into the colors of the rainbow, (figure d).
What is an acceptable and best for Polish and Symmetry levels?
As a rule, a «Good» grade by the GIA and AGS is «Good Enough». Improvements in higher grades is increasingly minute still, higher grade undeniably and unequivocally contribute to the diamond's ultimate beauty when it comes to perfection.
Igloo Diamonds ® are cut by highly trained cutters, and display the highest degree of facet alignment, closure, placement and surface polishing. Most are graded as having «Ideal» and «Excellent» Cut. Each of the Round Brilliant cut's 57 facets (!)
