The typically thick skin of most rough makes it much more challenging to peer inside. This skin can be easily removed in the cutting process, but dealers do not allow partial polishing (or windows) before a purchase. If inclusions are visible through the skin, it is very likely that the stone will finish Very Imperfect. Basic idea: If it looks bad with the skin on, it will usually look much worse with the skin removed. The obvious approach is the conservative one of assuming the worst, and (hopefully) being pleasantly surprised when you find better clarity than expected after the stone is finished.