Crystallization

Crystallization is a recurring phenomenon throughout the Final Fantasy series. It refers to a process in which a subject or being becomes a crystal.

Final Fantasy VI
In Final Fantasy VI Espers turn into magicite upon their death. Possessing magicite allows ordinary humans to use magic.

Final Fantasy XIII
Crystal Stasis serves as an important plot device in the story. When a l'Cie, a human branded by a fal'Cie, has finished their Focus, they will turn into a crystal for eternity. The appearance of a l'Cie's crystal varies. Most have their bodies naked and transparent, surrounded by additional crystal pillars. In one case, a l'Cie retains their original appearance (and clothing) and is simply frozen in a large crystal.

Final Fantasy Tactics
Crystallization is related to the battle system. If a unit remains knocked out for three turns (determined by the unit's Speed), their body will perish and become either a crystal or treasure box. (if said unit is undead, it has a chance of reviving with full HP instead) Once a unit becomes a crystal, any combatant who claims the crystal can either claim the battle abilities of the dead or fully recover their own HP and MP. If Ramza becomes crystallized however, it's game over.