User blog comment:PartyEnderman/No Mythril/@comment-24821572-20160310150642

Ores

"Only one of the hard mode ores will spawn, so multiple worlds might be necessary to get a certain ore"

If you have its alternative, then the counterpart won't spawn naturally in that world. In the future, reading some of the articles will save you the bother of wasting time, looking for something that will not exist.