A number of the Final Fantasy games center around the idea of four crystals, one for each element: fire, water, earth, air. The resonance between the four crystals structures the resonance between the four elements, feeds energy to all things, and allows life to flourish. When the crystals are balanced and allowed to emit their light silently and freely to all, life and harvest and all things good are in abundance, and the world is at peace. When something disrupts that balance, or a crystal falls under a shadow, life falters, the world slowly turns to wasteland, and monsters begin to appear and multiply in increasingly horrifying numbers.

If an evil person gets hold of the Crystals, they can usually do evil things with them, and there's always some kind of weird "unimaginable power" type thing that can be invoked by collecting all four crystals and being evil. This is usually where the plot comes from; the evil power is trying to get the crystals, and You Must Stop Them.

The crystals were used in Final Fantasy (the original), Final Fantasy IV (which also had four additional "dark crystals", the inverse of the four normal elemental crystals, which could be found on the underbelly the earth), probably in Final Fantasy II and Final Fantasy 3 (i think? i haven't played them), Final Fantasy 5, and in the relatively awful Final Fantasy: Mystic Quest. They kind of disappeared from the series after that point, and a good thing too, because although it's a nifty idea it would have gotten really old if they had used it any further.