I'm currently homebrewing some items:
The first one lets you grow small scales on your body to protect vital areas. Acts like the shield of fortification spell. Should this item have abjuration or transmutation aura?
The second one lets you harden your skin to make melee attackers reel from resonating vibrations. Acts like the jarring armor enhancement. Should this item have evocation or transmutation aura?
Give me a rule of a thumb - should I base auras on spell effects emulated or on their general flavor?
The first one is a clear transmutation.
The second could be either. I'd lean towards evocation because hitting someone in normal full plate doesn't have the same effect, the resonance must be enhanced by magic.
i.e. the mode of operation should affect the aura IMO.