Further to what Fallenchar said, realistically you can get away with zero crystal. It makes it a bit tougher, and you won't be able to equip troops with even the low tech magic items (like to ring of embers +1 def/+1 fire attack) or burning axe. You can still buy the stuff at the shop for your sov/champs, just will have to "make do" with mundane troopies and no mage staffcasters. I've played enough resource poor maps that I know the game can be won without crystal, but I always find it worthwhile to exploit crystal crags when I can. As well, if you are lucky enough to have a crag near your start position, that's a nice leg up - crags, even with enhancements, are really slow on the resource generation (edited to add) in comparison to the costs for any magical enhancements.

Anyway, it all depends on your play style and what you want to go for as to armies of doom or whatever.