Pierce is an allowed soulstone (if i am not totally wrong).
Armor Crush ( a soul stone with same effect BUT higher chance of activating) is the soul stone you're refering to, and Armor Crush stones (and if pierce is removed then that to) that was put into weapons before the removal from the forges, is still there TO. This is announced to be removed (Armor Crush Soulstones) in the next patch^^(for the sake of it :'(my sins grinding daggs *jumps after them down a cliff*