You'll get a Niveot Structure after 50 fuses of uncommon (green) crystals.
You need 3 crystals of the same kind (weapon / armor) for 1 fuse. They may be different type.
Then, you may fuse 3 Crux crystals with Niveot Structure to get Niveot Crystal (uncommon lv 65).
From combining crystals you may get Niveot Structure only once at each character. It will also earn you 3 archievements :D
I hope it was helpful.
~enjoy.