More common materials, on the other hand (iron, bronze, steel) would carry the risk of being completely damaged by blocking, and potentially harming the blocker with metal shards and a jarring impact.
Old melee weapons were generally made to be pretty sturdy, since they were expected to be clashed against each other in the first place. Part of the issue, though, is blocking vs parrying. Some of it is failure of animation; much like the lack of a dodge for when you missed in Morrowind, the way you can directly "block" with a dagger in Oblivion and have your enemy bounce backward is ridiculous. While it shouldn't be so easy to block with a weapon, it shouldn't be used for blocking so much in the first place.
It brings to mind another old suggestion on combining player and character skill. As someone would need to aim the crosshair for targeted damage, the idea is that when blocking, you don't just hold a block button and have it handle it for you, but aim the crosshair at the enemy's attack, directing your shield or weapon toward it. Higher skills would increase the effective radius around the crosshair that counts as a safe block. Blocking and parrying could potentially be tied into this via timing. The closer you are to pressing block at the moment of impact, the better the parry; the earlier you start holding it, the more it's just considered a full weapon block. Weapon blocking carries greater risk of staggering, disarming, or damage to the weapon, while effective parries open up chance for counters, smooth defenses without being slowed, or disarming the enemy. Like the radius, character skill would probably give a larger safe time zone.
It would also really separate shield blocking and weapon parrying as two different things, instead of the Oblivion method of casually stopping a hammer cold with your sword. Using a weapon defensively is about skillfully controlling the fight, changing a strike's path enough for you to move aside and let it pass harmlessly. Shields would be slower (another past idea was to have the crosshair in block mode move more slowly based on shield weight vs strength and skill), but would also be safer, having a wider safe radius around the cursor depending on its size/shape, and being more capable of completely blocking attacks and staggering enemies.