I'm thinking either one missing section (the last one and again, the breaking point should not be clean at all) or some sort of scratch that goes along the hand, making an opening in the armour, not actually breaking it off.
The way she is posed right now, if you removed the armour of the whole hand it'll look like she did it herself and that would make even less sense. xD
~I tried looking up broken armour but the closest I could find is a pretty damaged Iron Man mask that illustrates my point about not having a clean edge very well.~ Then again, it's up to you.