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The GIA assesses a diamond's polish grade under 10x magnification, recording all surface flaws and features to assign a grade. The fewer and less severe the polish defects, the higher the polish grade. Regal advises considering diamonds with Excellent to Very Good polish grades for the best sparkle:
- Excellent: This diamond exhibits no polish defects, or any tiny imperfections are visible only under magnification and with difficulty.
- Very Good: The diamond has extremely minor polish defects, such as faint scratches or transparent lines, which are visible only under magnification.
- Good: Minor polish defects, like moderate to heavy lines or scratches, are present. These may slightly affect the diamond's luster.
- Fair: The diamond shows polish defects that can be visible without magnification, such as heavy lines or facets that are worn. The luster is noticeably impacted.
- Poor: Major polish defects, including visible dark lines, deeply worn facets, and a lack of luster, can be observed without magnification.
These grades provide a comprehensive evaluation of the diamond's surface finish and its impact on its overall appearance and sparkle.
In conjunction with diamond symmetry, the characteristics of diamond polish play a role in determining the overall cut grade of the diamond. Both symmetry and polish are indicative of the quality of the diamond's finish. To attain an overall Excellent cut grade, the diamond's polish and symmetry must receive Very Good or Excellent grades. Similarly, a diamond with an overall Very Good cut grade will have symmetry and polish grades of Good or Very Good, and so forth.