Crystallization

Crystallization is a recurring phenomenon throughout the Final Fantasy series. It refers to a process which a subject or beings become a crystal.

Final Fantasy XIII
Crystallization serves an important plot device in the story. When the L'Cies, the humans who were branded by the Fal'Cie, has finished their focus, they will turn into a crystal and live immovably, eternally. The apperances of L'Cie's crystal vary. Most have their bodies naked and transparent, additionally surrounded by crystal pillars. In one case, one L'Cie retains exactly his original appearance (and clothing) and simply frozen in a large lump of crystal.

Final Fantasy Tactics
Crystallization is related to the battle system. When a unit is knocked out unconsciously for three turns (determined by the unit's Speed), his body will perish and become either a crystal or treasure box. In the case of crystal, any combatant who claims the crystal can either succeed the battle abilities of the dead or fully recover his own HP and MP.