Why are they collectively called Stevens anyway? None of the examines who go by their original name have that name.
In any case, some bellspawn ideas, in order of Bell appearance in Effulgence:
Shell Bell: sand grains
Sarion: Elvish for smallmages would be confusing; drakes could work for Ansharil
Phoenix or Raven: feathers
Stormy: clouds, droplets (unlikely to be useful if Effulgence wiki is accurate in reporting her as asexual)
Edit: I missed the in-story starlings for Stella; those work much better. I think sand grains work better than something molluscy for Shell Bell’s kids, though, because those can be the product of shells and also connect them to Pearl