it is luck. treasured increases the chance a little bit, and I think the luck affix does too. I have gotten a few crystal items from the shop, but I don't buy stuff in the shop very often, not enough gold to make it worth while. you have the same chance to get a Crystal Legend from the Codex as you do from a monster/breakable drop, a Feat Reward, or the shop. there is also a Crystaline Set Affix that increases the chances a little bit for a Crystaline affix on an item.