Exchanging crystals with runes

[mcgriffin]

Hi, I want to make exchanging magic crystals with runes (all kinds) an option in the shop in front of the dwarf capital. I guess this should be scripted in one of the .lua files but can't find it. Does anyone know how to do this? Many thanks in advance.
Edit: the ratio may be set as 20 crystals -> 1 rune & 1 rune -> 5 crystals so as to make alchemy worth taking but not overpowered.

[Habadacus]

Seems like this would be unbalanced in favor of the warrior who is swimming in extra crystals compared to the mage.

[mcgriffin]

then make the ratio for warrior/paladin/mage different, such as warrior 40:1, paladin 30:1 & mage 20:1. first thing I want to know is how to do it