I have a theory that on every Shardworld, there are two methods of writing. One can be read--and changed--by Shards and one cannot. This explains the artisan's script on Nalthis, which consists of colored dots, or colored words, as well as Aons as a form of writing in Raoden's study books. It is the basis for my personal theory of Roshar's focus being symmetry, since the glyphs are and must be symmetric. This breaks down on Scadrial, where words are just inscribed in metal to make them invisible. I think that maybe the metal symbols are also invisible to Shards?