Yup never compleated that quest crystal always breaks...so gave it up after many tries just like the orb quest in cruma.
Just abort the quest dump the knife & broken crystal and talk to him again he will give you a new knife & crystal if you want to try it again like i said never was able to compleat that quest gl
I remeber i had delivered a lvl 4 purity crystal for Cybellin once, she gaved me only 5,000 adena for it... made me so dissapointed. Quests with such silly rewards doesnt make sense.