The Collaborative International Dictionary of English v.0.48:

Intransmutable \In`trans*mut"a*ble\, a.
   Not capable of being transmuted or changed into another
   substance.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "intransmutable":
   changeless, constant, immutable, incommutable, inconvertible,
   indefeasible, inert, insusceptible of change, invariable,
   irretrievable, irreversible, irrevocable, lasting, noble,
   nonreturnable, nonreversible, permanent, reverseless, unalterable,
   unalterative, unaltered, unchangeable, unchanged, unchanging,
   undeflectable, undeviating, unmodifiable, unremitting,
   unrestorable, unreturnable, unsusceptible, unvariable, unvarying