The Doctor: You can't kill a stone.
Bella: No, but you can break it into little pieces and then look away so the former stone will have time to bleed to death.
The Doctor: This stone doesn't break like that.
Bella: Since when does stone not break?
The Doctor: Look, people think that things make sense but when you look at it from a not linear, non-subjective standpoint it's all just this big ball of wibildy wobbly-
The Doctor: Yes. Plot. And the plot says you can't kill a stone.
Bella: So I should just leave my chisel and mallet at home.
The Doctor: Why do you have a... no, I don't want to know.
Bella: So, in conclusion...
The Doctor: Yes?
Bella: If they did turn to stone they'd be incredibly killable, but they don't turn to stone, they turn to something else that is for whatever reason unable to be broken in a way that would kill the angels, and we just call that something else "stone" because single syllable words are convenient. Is that about right?
The Doctor: Yes.
Bella: And you have a British accent in spite of being from Golgafrincham, wait, that's not right, Gallifrey why?
The Doctor: Why does everyone care so much about my accent?
(Seriously, how many times did we have to hear, "Well, plenty of planets have a north."?)