Weapons

Bone, Wood, Stone, and Obsidian weapons break if you roll a Natural 1 (Critical Failure) on your attack. This includes high-tension weapons like bows.
Certain weapons are particularly brittle (such as Obsidian) and also break on a Natural 20. The attack is still considered a Critical Hit, and does full damage, but breaks upon impact.
Some weapons are immune, or have increased resistance to breaking, such as the Elven Volante, or certain high quality Dwarven weapons, or very simple weapons with no impact issues, such as slings and Atlatl's.
Crafting an edged weapon out of Obsidian makes it a +1 weapon, although it is not considered magical. Obsidian weapons are also considered Brittle, and so they break on a Natural 1 and 20.