Stoneskin

Stoneskin is a returning boss modifier in Diablo III. This is one of the few modifiers which are directly carried over from Diablo II not only in name but also in function.

The modifier greatly increases the boss' resistance to physical attacks, which leaves them fairly wide open to a magical assault. It is unknown if the monster's defense with stoneskin will be gauged by the defense attribute which reduces damage by a flat rate, or if there is a separate system set up for calculations.