How did C.C get her scar? In Code Geass, when we see C.C mainly topless we see that she has a scar on her left breast.

The first time we see this is when Lelouch tried to get C.C to move away from the Lancelot when she's feeding Suzaku shock images to destabilize him. when Lelouch touches her he sees her memories and there is a scene where C.C is naked in a river and there is a close shot of her scar.

However in the scenes were we see when C.C obtained the Code I can't really see the scar on her when she's in the church naked talking to the nun who gave her Geass. After C.C is covered in blood from killing the Nun who gave her Geass i still don't see the scar.

The scar itself kinda looks like a trident and not the Geass Sigil which questions if it's even linked to Geass itself. So i am wondering, when and how did C.C get this scar? also given how she been shot in the head and stabbed with shrapnel and healed completely how come the scar hasn't healed as well?
Firstly, C.C. did not kill the Nun. The Nun's intent was to force C.C. to take her Code, and once C.C.'s Geass matured, attacked her to make her do so, and then killed herself. C.C. receives her scar from the Nun, during that altercation. 

Presumably, the Nun tried to draw a Geass symbol, but it was done imperfectly. 

Presumably, the scar persists because it was one of C.C.'s features before her death, so it doesn't go away completely.