Artifact for healer on Nefer teams

So im using Nefer (unveiling), Lauma (silken), Xingqui (deepwood) and Diona C6 (Noblese). What artifact should i use on Diona since i know Noblese doesnt do anything for Nefer and i do not own a complete instructor set. Also does building crit on Lauma serve a purpose in this team or should i go full EM build.