There are many grades of diamond polish but most laboratories use the GIA polish scale:
Excellent: You cannot even see any polish lines with a 10X magnification loupe. The diamond is flawless on the outside. You should know that pits, nicks, scratches, bruises, parcel chap and other surface blemishes, may not affect the Polish grade of the diamond but may be mentioined in the Comments area of the Certification. Some Laboratories call this grade Ideal, but that is not to be confused with the diamond cut grade of Ideal.
Very Good: Has a few polish lines that can be viewed through a 10X loupe, but cannot be seen with the naked eye. This grade will save you the premium that you would spend on a Grade of Excellent, but will for all practical applications, look absolutely the same.
Good: This grade provides the most bang for the buck. There are many polish lines on the diamond, but you will still need 10x magnification to see most of them. Occasionally one or two of these polish lines may be visible to the naked eye, but they are still so small that they are not apparent and since they are clear in color and in between facets, they will usually blend right in nicely. This is the lowest grade we like to see people go with.
Fair: Several Polish lines are eye visible. They are still not very noticeable since they are camoflaged between the facets. At this point the diamonds brilliance, Sparkle and Fire will start to be affected, to a small degree.
Poor: Has many eye visible Polish lines. You can see the white lines between the facets if you look closely. This affects the Brilliance, Sparkle, Fire and overall beauty of the diamond. In some cases, the diamond may look slightly cloudy as a result of the gap's between the faceted surfaces.