Polish is graded the same way as
symmetry: Excellent, Very Good, Good, Fair or Poor on a GIA report.
Poorly polished facets may reduce the intensity of light reflected
from, or refracted into and out of, a diamond. Labs assess polish
by examining the diamond, facet by facet, with reflected light
under a microscope.
The market has read more into Polish (and symmetry) simply because
it is there on the report and because GIA has no cut grade standard.
A common polish defect is surface grain lines. Even the most skilled
cutter can encounter variations in hardness or grain, just like
in wood, as they polish a facet. The result is a microscopic polish
line running across a facet. These grain lines are very common
in pink fancy colored diamonds, but often they can only be seen
in reflected light.
If you choose a diamond with an SI or VS inclusions, a few microscopic
polish lines may be of no relevance. But if you were considering
buying a Flawless diamond, then excellent polish could be a major
consideration.
If the polish is rated as fair or poor, visual performance may
be noticeably reduced.
