Quantum-locking

Quantum-locking is a feature gained by species through evolution. The only known species to evolve this ability were the Weeping Angels.

If a creature is quantum-locked, then they are turned to stone whenever they are observed. However, if the observer turns away, blinks or stops observing them in any way, the creature can move for the time until the observation resumes. This is used as a defence system. Between observations, Weeping Angels would attempt to attack. The Doctor said that 'You can't kill stone,' this is highlighting that quantum-locking is one of the most perfect manners of self-defence.

If two creatures are quantum-locked and are facing each other, they must be careful to cover their eyes before they are turned to stone. If they don't, they risk looking at each other, and being that they are both being observed by the other, they would be trapped in stone form. (DW: Blink)

It is at one point comically remarked by Larry Nightingale that one could "chisel their heads off" when in stone-form. Whether this would work or not is unknown, but one can imagine that the being, once in living-form again, would end up dead and headless. This also brings up the idea of whether or not blood would turn to stone when looked at, but once again it seems almost obvious that it would. During The Time of Angels, one of the Church soldiers fired upon an Angel in stone form. It is unknown whether or not this harmed the Angel, or what happened to it later, though it did cause damage to the stone itself. Whether this effects its living state or not unknown, as the specific Angel is not seen again.