Using Crystals
I'm very new to the game... just started over the weekend.
I have a crystal source next to my city, and I can't decide whether or not it's worth it to spend multiple turns building the necessary upgrade to use it at this point in the game (my city just went to level 2, so it's pretty early). What exactly will I get from crystals? I checked the in-game encyclopedia, and it mentions the thing you build, but I can't find anything about the resource itself.