So me (XDogzZ) and Imbadboy6 were mining runite ores in World 2 when all of a sudden, one of the runite ores turned into a coal ore, it turned into a coal ore for me 5 minutes later as well.
It seems that 3 of the runite ores down in the mines can become a coal ore at one point, and not only are they visually coal ores, when they are mined then its collected as a coal ore as well. They turn back into coal ores randomly.
See for the attachment down below for an example of one of the "new" coal ores. Suggest looking into it and have a nice day/night!