Materia to Guardian Forces to the Sphere Grid and plain ol' leveling up, the series has introduced some novel forms of strengthening your characters. It sounds like Final Fantasy XIII will be no different -- according to the latest issue of Shonen Jump, the RPG introduces a new progression system called "Crystalium."
Andriasang laid out the gist of how the system operates -- instead of gaining experience from battles, players earn "Crystal Points," which they can then spend on new abilities and stat boosts which are arranged on a circular chart. As you progress down one of the chart's specialization trees, you unlock stronger abilities; Fire leads to Fira, and so on. Sounds a lot like the aforementioned Sphere Grid -- but we suppose we'll have to wait until we get our hands on the game before we can make a more insightful assessment.