loking from the other side.... fairies are thought to be created from the imagination of mortals. So technically anything your magi can imagine can exist, as part of the fairie relm. So you should then beable to creo anything you desire.
Yes, but just because one person imagines or dreams about a thing, doesn't mean that such a thing will suddenly exist in the faerie realm. But if all the villagers start talking about some mysterious event, and start explaining it, this thing might become real.
Since there are floating rocks (which we today know contains air), mediaeval mindset would allow for rocks floating in the air... So - magi can create them...
Mind you, you might be looking at very high complexity, and probably non-permanent creation...