I found this at the Nihilium thing posted some time ago.
"Stigma...sweet name that will remind some of us, the materia system in FFVII, explanation : armor isn't the only way to differentiate characters, there is the Stigma system! Every character around level 20 will be able to use some special "stone" named stigma, each character got 5 slots for them, so it's not something you put on your gear like gems and their are class specific (there isn’t 5 slot at level 20 though, every 10 levels one slot is unlocked probably). The only thing we have seen is that stigma brings some "training" allowing character to use shield, plate or halberds...(there is a stigma for each types of weapon and each types of armor but they differ whereas you’re fighter or priest) Perhaps in the future we will see some classes specific skills, being available to some other classes through the stigma system ! And hm... you can purchase stigma through a NPC, price are quite high : 200 000 quina (=gold, but it's not the same scale a WoW of course...)"
So it seems u buy ur stigma, but i guess rare uber ones will drop at low low low rates
