Cornerstone is a shield in Dragon Age II.
Acquisition
Looted off the body of Jeven during the quest Favor and Fault.
Trivia
If Aveline is equipped with this shield, bumping her through impassable obstacles like a flaming barricade will not knock her back (thus, proving the shield's ability). Instead, she always screams in pain. As for Hawke, though he/she cannot also be pushed back, he/she will not make any sound that he/she is hurt.