Well, i havent read the manga or anything, but I dont think his earring means anything. There is a character system in manga called the Star System (anyone who reads Shonen Jump, you know what I mean). The Star System is where you take characters from another anime/manga and put that character into another one. I know the Star System is only characters but maybe its for objects as well. In Shonen Jump, they talk about the Star System in their interview with the creator of Shaman King. One of the characters from Shaman King, Anna, is used in another manga called Butsu-Zone (or something like that).