User blog comment:IamJakuhoRaikoben/Chapter 361: The Two Bombs, Review./@comment-5668006-20131123040415/@comment-24066070-20131123102224

@FTGM: I know about that. But so far we know next to no details about those demons. Gray identified Doriate as a demon from the book of Zeref, but it was confirmed that he was once human.

Furthermore, I doubt Minerva would lose all her magic and just get a "curse" instead.

So it might be possible that some demons where once human and retained their magic (not wanting to bring up the whole "Silver is Gray's dad" thing again)

We don't even know what the Book of Zeref is, but Hades created demons from pebbles. So it could be the book contains details on how to create a demon out of something else, whne this is used on humans you get Doriate, Minerva and possibly Silver.