Quantum-locking

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

Mechanics
If a creature possessed the ability to be quantum-locked, then it would transform into stone whenever it was observed. However, if the observer turned away, blinked or stopped looking at the target in any way, then the creature could move until the observation resumed. Between observations, Weeping Angels would attempt to attack. However, quantum-locking was used as a defence system. The Doctor once said that "You can't kill stone," highlighting that quantum-locking was 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)

Harming
It was at one point comically remarked by Larry Nightingale that one could "chisel their heads off" while a Weeping Angel was in stone form. (DW: Blink) Whether this would work or not is unknown. Once, a Church soldiers fired upon an Angel when in was in its stone form. It is unknown whether or not this harmed the Angel, though damage was caused to the stone itself. (DW: Flesh and Stone / Time of Angels)