What properties would an ideal ("perfect") diamagnet necessarilly* have? (*as in in, implied by definition.) This is a spin-off from this thread, based on the common idea that superconductors are better described as perfect diamagnets than perfect conductors. If it began in zero magnetic field, a perfect diamagnet should completely repel any new field from entering. But if the material transitioned (perhaps smoothly with temperature, say) into the perfect diamagnetic state from some other phase which is only weakly magnetic, would it actually exhibit the Meissner effect (completely excluding a pre-applied magnetic field)? And would it be superconducting (I saw a three line argument that it could not have finite resistance, but could it just be an insulator)?