To my understanding, the arcing is caused by hard edges. E.g. all the elections could be at the end of a fork's tines, and EM field forces them to jump to another tine instead of going through the root of the fork.

Since spoons are rounded, they don't need to jump. I don't think the material plays much of a role in arcing other than providing resistance. They would heat up, but they'd melt before they arc. Still, they can arc from one spoon to another spoon when there multiple spoons close enough.