Crystals and Philosopher's Stones
Just remove soulbound from the game.
You buy it with soul-bound currency, why would it be more logical to have the result of the purchase account bound?
Why not make skill points account bound and be done with it? and while you are at it, character experience, and map completion, and anything else that is still left for a reason to play another character.
Augur stones are soulbound, they can do it.