I think the helpfile says whips, flails and daggers can't be weaponbroken.

Legend has it there are certain materials that can't be weaponbroken, regardless of what the weaponbreaker's axe is made of.

Certain extremely rare, buff weapons can't be broken.

Weapons containing other peoples' souls can't be weaponbroken.

If you're curious about how certain materials stack up against other materials, ignoring the above examples, I wouldn't hold your breath for an "official" answer. Too crunchy.

If the formula remotely resembles real life then wood would be the weakest, followed by gem/stone, then probably bronze, iron, steel, titanium, mithril and adamantite. Maybe swap the last two. It's hard to say how brittle various fictitious metals might be. And it's not just about hardness - glass can scratch lead, but lead's alot less likely to shatter than glass.

One idea for weaponbreak - give the weaponbreakER's weapon a chance to shatter if its material is inferior to the material of the target weapon. If you're trying to shatter my adamantite polearm with your opal-bladed axe...maybe the opal loses. Positive spin: makes weaponbreak more tactical, albeit less useful overall.