The polish grade rates the skill of the craftsmanship applied to the outside of a diamond.
The polish grades from “poor” to “excellent”, and is a measure of the quality and skill used by a cutter as he finished the Facets on a diamond. Good polish is crucial for maximum Brilliance of a diamond, but it takes a trained eye to distinguish between polish grades. Extra Facets to remove a part of the Rough Diamond are common, as well as grainy Polish Lines that are visible only under a Loupe or microscope.
The quality of this workmanship is important to the overall brilliance and Light Performance of a diamond, because it affects the reflectivity of the facets, and thus the flow of light inside the diamond so it can be reflected back up to the eye through the crown.