TLDR I may or may not make the switch to a physical spearzon, but I'm wondering if such a build would have any use for a protector stone.

I just learned that the ed/damage jewel bug still exists for helms and body armor (anyone can confirm?)

And so it would need to be in a weapon, but a physical spearzon would only use a runeword weapon right?

This seems obvious to me but I'm double checking before I pass up on a very nice protector's stone

There is no bug edmg on helm and body armor are calculated differently and it is not as strong as edmg on a weapon. The adds 10-30dmg and -phy res is why someone would use the stone

I understand that edmg is calculated differently on helms/armor and on weapons.

This is an old and well documented bug though, if you're saying that the bug has been fixed that's one thing, but the bug very much did exist in the past.

https://us.forums.blizzard.com/en/d2r/t/enhanced-damage-minmax-damage-bug-protectors-stone-jewel/172003

As far as i know, there hasn't been any reliable evidence that the bug has been fixed yet.