When I was originally coming up with the idea- the dark djinn was a result of the crystal there taking over a physical body and using it for it's own means. In my mind the crystals are sort of semi-sentient and always working towards some unknown means. In that vein the good or 'light' djinn have harnessed the crystal's power without being entirely taken over by it, or something. Honestly I'm still kind of fleshing all that stuff out. The main thing is when you put the crystals into your physical self, you no longer have agency in your actions, but are powerful as hell.