Like many Jews I have the minhag of placing small stones on Jewish graves I visit. (The precise roots and meaning of this custom have many explanations, but it is widespread and I am not questioning it here. I saw my grandparents do it. I saw my parents do it. I do it. See for example Placing stones on a grave )
My sister was well-known for her habit of putting one color of nail polish on nine nails and gold or silver on one pinky nail; she kept her nails painted that way even as she was dying. When we unveiled her tombstone I placed a rock (small, perhaps 3 cm long) that I had painted half pink and and half gold with some of her own nail polish that I had saved.
I have never seen or heard of decorated rocks being used in this way before; it was just an idea that came to me. Are there any halachic (or later) sources that talk about requirements for these stones?
(See also Where should the stone left on a grave come from? asking about the source of the stones, but my question feels rather different.)
I realize that parts of this question are going to attract opinion-based suggestions, please keep your opinions to comments. For answers I'm looking for sources to Jewish authorities of any flavor who have written something that might bear on this issue.