Well all I know for sure is that Terry Pratchett claimed that a Wizard's Staff has a knob on the end
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I am temped to agree with Formy regarding the authority to 'practice magic' maybe resided in the staff, as I read it Saruman was deprived of his magical power, apart from his hypnotic voice, when Gandalf broke his staff.
I wonder if a proportion, maybe even a majority, of an Istari's power did reside in their staff, or require the staff to allow its use? Could this have been one of the limitations that was imposed by the Valar when the Wizards were sent?