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I would say that it would bypass stoneskin. Normally if you are talking about hitting a creature that can only be hit by a +x weapon, I would say it would be the same with stoneskin. Per MOF pg 141. "...harms creatures with DR as if it had a +5 enhancement bonus". And if you look in the PH pg 257 for stone skin it states "... though a weapon with a +5 enhancement bonus or any magical attack bypasses the reduction).