On close inspection, the brooches change expression constantly, always mimicking Zeetha's expression. They actually reflect the mood of the wearer of the headband: they are worn in Skifander as a mechanism to show the wearer's honesty.
If the headband is placed on someone else, it will reflect their mood -- if they are secretly nervous or lying or sad, the gem will show their true underlying expression. (To the wearer: be honest, especially if you have something to hide. Say what emotions the gem is showing.) This is not something done lightly -- even in Skifander, these gems are rare and precious, and it is an honor to wear them. But they can be used as a tool if needed.
Characters In Game: Zeetha
All Characters: Zeetha
The following were notes made while this was in development. They do not necessarily represent the final state of the thing.
Zeetha has several gems on her, which look like smiley faces: they show a simplified expression. They always reflect the current mood of their wearer. In Zeetha's case, their expression always mirrors her, because she does not particularly go in for that sort of deception. But if someone wears them, and is trying to conceal their mood, the gems will show what they are truly feeling at the moment. If asked, the wearer must honestly admit what expression the gems should be showing right now.