Polish influences how well light is able to pass through a diamond. It is a very important attribute in determining a diamond's overall brilliance. When choosing a diamond, it is best to pick one that is laboratory certified with good, very good or excellent polish. Diamonds with poor polish are significantly less brilliant because they have microscopic polish lines that blur the surface of the stone—this also reduces the amount of light that enters or exits the diamond. Be aware that many diamonds have a poor polish because some diamond cutters reduce their costs by not spending sufficient time to properly polish a diamond.
Polish is graded the same way symmetry is. On a GIA report, the grades are Excellent, Very Good, Good, Fair or Poor. The same applies for AGS and IGI reports, which also include ideal in their list.
There is very little difference between these ratings to the unaided eye. A diamond with a polish rating of Good can still be a remarkable stone. It is only under magnification that the differences in these ratings become clearer. Polish rated Fair (FR) and Poor (PR), however, indicates that the flaws in the polish are visible to the naked eye and affect the overall beauty of the diamond.