Crystal
Crystal is one of the four magical resources (along with Gems, Mercury and Sulfur). Crystal is needed to construct several buildings and to hire Green Dragons, Gold Dragons, Ancient Behemoths and Crystal Dragons. The primary source of crystal is the Crystal Cavern, which produces 1 unit of crystal per day for its owner. You can also find crystal scattered around the land in small amounts (3-6 units of crystal per resource stack).
In Rampart towns, you can also build a Resource Silo which will bring you 1 unit of crystal per day.
Sephinroth the Warlock is able to produce 1 additional unit of crystal per day for the Kingdom she serves. Also, any hero equipped with the Everflowing Crystal Cloak gives you 1 unit of crystal per day (Sephinroth would produce 2)