So I'm working on a homebrew Swords & Sorcery setting which features the "13 cities of steel".
These are 13 city states that have mastered forging metal weapons and armor.

The lands outside the cities are populated largely by savage sub human hordes or primitive stone age tribes. 

I've been trying to figure out a simple way of reflecting the difference between metal weapons and stone weapons.  I'd rather have something other than a simple bonus die to combat. And I imagine that for the purposes of a game, getting hit in the head with a stone axe will do largely the same sort of damage as getting hit in the head with a metal one, so damage modifiers are not terribly interesting to me.

I'm wondering if there's a way to represent stone weapons as being more fragile than metal ones.
Maybe they break when the user rolls a double on the attack?
It's not something that will make stone age weapons useless, but it certainly makes metal weapons far more desirable, right?

Any thoughts would be appreciated.

Alex