You do have to bear in mind that Dumbledore is quite crazy (in a purposeful sort of way). It wouldn’t surprise me if he simply found a rock that James Potter had kicked around for a while, and on that basis claimed it was “James’s Favorite Rock”. Then the rock became a way to force Harry to improve his Transfiguration skills in a huge way.
Also, as rdb notes below, that kind of concentration simply must have bleed over into other types of magic.
Plausible, but that can’t be the only reason for the rock; too big a deal was made of it at the time.
You do have to bear in mind that Dumbledore is quite crazy (in a purposeful sort of way). It wouldn’t surprise me if he simply found a rock that James Potter had kicked around for a while, and on that basis claimed it was “James’s Favorite Rock”. Then the rock became a way to force Harry to improve his Transfiguration skills in a huge way.
Also, as rdb notes below, that kind of concentration simply must have bleed over into other types of magic.