There’s got to be an origin for the idea that X always marks the location on the map where the buried treasure is.
The question is, is it a real origin, in that there really was a map of buried treasure marked with an X?
It certainly is realistic: a monochromatic map done with ink and parchment wouldn’t use color to denote objects, but symbols. A big fat X is as good a symbol as any, provided you can tell it apart from a simple intersection of two lines; and it requires no literacy.
Or is this a fanciful origin, coming from a particular book or film?
Well, it’s not in the early treasure-hunt stories the Gold Bug or The Musgrave Ritual. For some reasion, I’m thinking of Treasure Island, too, but I can’t recall if it’s actually in there.
““I’ll tell you what I’ve heard myself,” continued Captain Smollett: “that you have a map of an island, that there’s crosses on the map to show where treasure is, and that the island lies—” And then he named the latitude and longitude exactly.”