From Granblue Fantasy Wiki
Weapon Stones are rare materials needed for advanced crafting. Players typically get them from reducing unneeded and weapons.
|Uncap Celestial weapons to 4★||20|
|Uncap Omega weapons to 4★||200|
|Uncap Showdown SSR weapons to 4★||60|
|Rebuild Class Champion Weapons||256|
|Change a Class Champion Weapon's element||512|
Weapon stones are primarily obtained from reducing weapons.