As far as I realized, every new skill unlocks a new quest, where the stone basically explains you how to use that skill.
I unlocked Crystallographie yesterday and did not get any quest. I first thought that maybe those are only for the beginners, and you get less quests the more you climb up levels, but for the next 3-4 skills I unlocked afterwards, I always had the stone talking to me.
I checked the questlog again, and there is no Crystallographie related thing.
Bug?