The Obsidian Graver. Won’t actually cut or pierce any physical objects but will leave irreparable gouges in ongoing magical effects or the enchantments on items. Can be used to leave symbols or messages that can only be viewed when Detect Magic is cast so you can see where the marks are. Keep at it long enough & you can eventually whittle away part of an enchantment – carve a hole through an anti-magic sphere, cut a weak point in magic armor, leave two foot-sized mundane spots in a magic circle that you can stand in & say “Look, there’s nothing here at all, it’s safe, come on over”, & so on.