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The polish grade is a measure of the visibility of polishing lines on the surface of the diamond when viewed with 10X magnification or the unaided eye. These polish lines are caused by microscopic diamond crystals pulled across the diamond facet by the polishing wheel during the diamond cutting process. Polish lines are similar to the streaks left on a car after it is waxed.

Polish can include minor blemishes (scratches and polish lines, burn marks caused by excessive heat, rough girdles, lizard skin, nicks and pits, abraded culets and facet junctions) viewed with 10X loupe or microscope but that do not influence the clarity grade.

Obvious polish lines, polish marks on several facets, or noticeable blemishes put a stone in a fair category. Moreover, polish lines, polish marks, or blemishes that reduce transparency call for a poor rating (a situation usually experienced in low-clarity stones). When buying a diamond, always seek at least good polish. While diamonds with very good and excellent grades for polish are desirable, there are fewer of them, they are generally more expensive, but often not noticeably better when viewed with the unaided eye than diamonds with a good rating.

The polish grade refers to the amount and visibility of the polish present on the diamond surface. There are five polish grades.

  • Excellent: No polish lines or marks visible when the diamond is viewed under 10X magnification. Excellent means “superior,” not perfect. If a few hard-to-find polish lines or tiny blemishes are very difficult to see visible under magnification, the diamond can still get an Excellent grade.
  • Very Good: Minor polishing features visible face-up when viewed under 10X magnification. This includes stones with only faint polish lines or insignificant blemishes in inconspicuous places.
  • Good: Numerous minor transparent lines visible under 10X magnification. Luster of the diamond is generally not affected when viewed with the unaided eye.
  • Fair: Obvious transparent, white polish lines or burn marks visible at 10X magnification. Luster of the diamond is affected when viewed with the unaided eye.
  • Poor: Prominent polish features are seen face-up at 10X magnification. The luster of the diamond is significantly affected when viewed with the unaided eye.

For fancy shaped diamonds, a lower percentage of diamonds have Excellent or Very Good ratings for symmetry than with round brilliant diamonds. Because fancy shapes are not as uniform in their facet patterns, it is more difficult to achieve the higher polish grades.