Polish lines are very common even on high color and clarity stones. If a GIA report documents "Good" or "Very Good" for polish, polishing lines have been detected to at least a faint degree. Even in an "Excellent" polish, if the diamond is studied long enough, chances are some very faint lines may be found. The higher the polish, the fewer the lines that will be seen on the surface of a stone. This isn't always easy for the cutter however. Sometimes he may encounter knots, or irregularities in the grain, that make cutting very slow and difficult, leaving detectable polishing lines. Frequently the cutter's scaife is not in the condition that it should be for the required fine polishing.
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.