A Stigma ( スティグマ , Sutiguma ) is an ocular condition with six variations, the Magic Eyes give their hosts incredible inherent magical abilities. Despite their increased magical power, or perhaps because of it, Magic Eye bearers are almost universally hated and feared in all known countries. Once a person is confirmed a bearer they and their close family and friends are killed.
While the majority of people refer to the Magic Eyes as "Demon Eyes," the bearers themselves have taken to calling them "God's Eyes." The three most common types seen throughout the anime series are: the Alpha Stigma, Lino Doue, and Ebra Crypt. Others like Torch Curse, Will Heim, and Ryner's Tear Stigma have only been seen in the manga and light novel thus far and not the anime, but are referred to.
Crystallization occurs by disruption of the user's eyes or mind, which drives the user to the edge of madness and beyond, allowing the crystal to be gouged out. Ryner Lute is the first case in which a user has been able to return to sanity after going on a rampage. This is possibly due to his half of the Lonely Devil inside him.
These crystal forms of various Stigma are then usable as items in combat, each crystal containing immense power within, as seen by its use primarily by the nation of Gastark. The seemingly callous methodology of mass producing these crystals as weapons of war is not lost of them, as Lir, Sui, Kuu (all dispatches from Gastark) are seen intent on doing the above as a secondary objective.