User blog comment:B214/Talk About What You Want! 3/@comment-99.101.204.248-20141127062737/@comment-16281343-20141201053055

A Devil cannot turn their pieces to Mutation ones. Rias has one on her set when she received it. IIRC, the chances to receive one is 10%.

At that particular time, only the mutation piece was capable of reincarnating Gasper. Even she didn't know why Gasper needed a mutation piece.