there's a pretty finite number of crystals you can get from a mission as opposed to gold, iron which drop from resources etc., and the crystal costs for resists can be pretty wild -- 22 for the last rank!! Plus it's shared heavily with building upgrades, enchants (also extreme, 30+ crystals to gamble)
#crystal costs for resist upgrades seem extreme
coral otter
subtle saffron
You can actually farm those pretty easily, just start a run with your strongest character, go to the dark citadel and lit all torches, when you clear the waves you get a bunch of trinkets, collect them, leave, repeat, it's kinda boring after a while but it's really easy and fast, i get about 30 and 8 red ones per run and it doesn't take more than 2 mins