Only by taking hold of a handbell can geomancers unlock the full potential of their magical munitions. Polishing one's skill in the handbell makes it possible to enhance the effects of geomancy spells.
This totally sounds like BRD's instruments. Without a bell (in the ranged or maybe sub slot) you will be able to cast Geomancy spell like a BRD is capable of casting songs without an instrument, they'll just be gimped in one way or another.

The orange looking things wielded by GEOs in the artworks are new models of clubs, not bells.