Polish Grade
Polish Grade
The five-point laboratory assessment of facet-surface quality on a cut diamond
Polish grade is the laboratory assessment of how cleanly the facets of a finished diamond have been polished, expressed on the five-point scale GIA uses for diamond grading: Excellent, Very Good, Good, Fair, and Poor. The grade is determined by examining facet surfaces at 10× magnification under standardised illumination, looking for polish marks, scratches, abrasions, and other surface irregularities left by the cutting process.
What polish grading looks for
Polish defects are categorised into a small set of recurring types. Polish lines are fine parallel marks left by the polishing wheel, visible across single facets. Surface graining is structural roughness arising from the diamond's own grain orientation during polishing. Abrasions occur at facet junctions, where the polishing process has rounded or chipped the meeting line between two surfaces. Burn marks are areas of surface damage caused by overheating during polishing, often visible as cloudy or hazy patches. The grader counts and weights these defects to assign one of the five grades.
The 10× standard is significant. Defects that would be invisible to the unaided eye are still counted toward the polish grade, because the grade describes the absolute quality of the finishing rather than the eye-visible appearance. This separation between absolute and visible quality is a recurring feature of GIA's grading system.
Why polish matters
Polish quality affects light performance through two mechanisms. Surface polish lines and abrasions scatter incident light, reducing the proportion that follows the geometric path the cutter intended through total internal reflection. Burn marks and graining roughness produce small light losses across the affected facets, slightly muting brilliance. The effects are real but small at the higher polish grades; below Good they become more noticeable to careful observation.
Polish is graded independently of symmetry, which describes the alignment and proportion of the facet pattern, and of the cut grade, which describes the proportions of the stone overall. A given diamond receives all three grades on its report, and the trade reads them in conjunction. Triple Excellent — Cut Ex, Polish Ex, Symmetry Ex — is the standard top combination for round-brilliant stones.
The grading scale
The five grades describe a continuum from defect-free to severely flawed:
- Excellent (Ex): no polish marks visible at 10×
- Very Good (VG): minor marks visible only at careful 10× observation, no effect on appearance
- Good (G): minor marks readily visible at 10×, no significant effect on brilliance
- Fair (F): noticeable marks at 10×, may slightly affect brilliance
- Poor (P): prominent marks at 10×, noticeably reduce brilliance
The thresholds between grades are calibrated by GIA against reference stones and grader training, and the same calibration is used by other major laboratories such as IGI and HRD, although terminology and exact threshold definitions can differ slightly between laboratories.
In the trade
For round-brilliant diamonds, polish is one of the inputs to the overall cut grade. A stone with poor polish cannot achieve a high overall cut grade regardless of its proportions, which is one of the reasons modern cutters aim for Very Good or Excellent polish as production standard. For fancy shapes, where GIA does not currently issue a cut grade, polish stands alone on the report as one of the principal quality markers visible to the buyer.