In the Gemological Institute of America (GIA) Diamond Cut Grading System for standard round brilliant diamonds, the aspect of craftsmanship — i.e., the care that went into the crafting of a polished diamond, as seen in its finish — plays an important role in the overall cut grade along with the other aspects of face-up appearance and design. The two categories that define finish — polish and symmetry — have been part of GIA’s International Diamond Grading System™ since its inception in the mid-1950s and are listed on each GIA Diamond Grading Report and Diamond Dossier®. It is important to note that with the introduction of the new cut grading system, the criteria for determining polish and symmetry remain the same.The following is a review of the criteria and the methods used for assessing them, and will explain their role in the new cut grading system. This article expands on the GIA’s course material for assessing polish and symmetry, describes the guidelines used by the GIA Laboratory in evaluating finish and presents specific examples of the different polish and symmetry categories. In the GIA Diamond Grading System,“polish”refers to the quality of a diamond’s surface condition as a result of the polishing process or to blemishes created after the cutting process, often referred to as “wear and tear”. Polish features are located on the surface and do not visibly penetrate into the diamond at 10X magnification. Polish is assessed on a scale consisting of excellent, very good, good, fair and poor. “Symmetry” refers to the exactness of the shape of a diamond,and the symmetrical arrangement and even placement of the facets. Symmetry is also assessed on a scale ranging from excellent to poor. In the GIA Diamond Cut Grading System for standard round brilliant diamonds, on the GIA D-to-Z color scale and Flawless-to-I3 clarity scale, finish is factored into the final overall cut grade as follows:
• To qualify for an excellent cut grade, both polish and symmetry must be very good or excellent.
• To qualify for a very good cut grade, both polish and symmetry must be at least good.
• To qualify for a good cut grade, both polish and symmetry must be at least fair.
• To qualify for a fair cut grade, both polish and symmetry must be at least fair.
• A poor cut grade is assigned when either polish or symmetry is poor.
To determine the relationship between finish and overall cut quality, GIA conducted extensive observation testing using standardized lighting and viewing conditions. Observations of diamonds with comparable proportions, but differing in their polish and symmetry categories, were analyzed to determine the effects of finish on overall cut appearance. In this way, GIA researchers found that a one grade difference between the other aspects of a diamond’s cut grade and its polish and symmetry assessments did not significantly lower a trained observer’s assessment of face-up appearance, and could not be discerned reliably with the unaided eye — e.g., polish and/or symmetry descriptions of very good did not cause observers to lower their assessment of a diamond that would otherwise receive an overall cut grade of excellent.
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