A few faint polishing lines are not going to affect the brilliance or value of a stone too much. “Very Good” and “Excellent” polishes on GIA reports can definitely improve salability and may also increase value slightly. “Fair to Good” (or lower polishes) can decrease salability and should discount a stone at least slightly.
The polishing lines that appear on the vast majority of stones you will look at, are also an excellent way to identify diamond. As discussed earlier, twelve individual grains travel through each and every piece of whole rough diamond. Even when part of the diamond is removed, the grain still influences the cutting position, something that the cutter must always keep in mind. Remember that each grain must be polished in a perpendicular direction for material to be removed. As a result, polishing lines will be found to go in several different directions on a diamond, as compared to the typical one direction in most other soft stones.