Then someone discovers how to mass produce it.
Would produce a lot of interesting effects in social upheaval. And social climbing. Even if it slowly and gradually seeped through society.
I wonder if the high-ranking families would try to preserve some forms of magic for those of high birth. It would only really work, of course, if there weren't rivals about, or if the magic was not really powerful. Otherwise you would have to allow it out in self-defense, to avoid losing the arms race.