The lower layer is polycarbonate around a mm thick and thus tough and can stand some polishing (radial!). The top is generally a blob of lacquer or polymer which is spun on the reflective layer to even out the thickness with the label silk-screen printed on. I haven't seen many reports that cd marker pens harm the top layer, but there are reports of certain label adhesives reacting with the screen or lacquer, presumably from those manufacturers who perform no ageing tests on their products. I don't use labels because it is very difficult to centre them manually and they cut down your reading speed as often as not if slightly imbalanced, or at least cause servo hunting.