Crystals are the "currency" in the game.
There are 5 types:
They are "equally valued" but every player is attuned to only one of them, meaning that you can gain that type a lot more easily, but you also need it more to spend when building and upgrading Facilities
They have nearby colors in some cases, meaning that for green the nearby colors are yellow and blue. Note that red and purple are nearby to each other as well.