Inconvertible

(a.) Not convertible; not capable of being transmuted, changed into, or exchanged for, something else; as, one metal is inconvertible into another; bank notes are sometimes inconvertible into specie.

Thesaurus Entries

changeless, constant, immutable, incommutable, indefeasible, inert, insusceptible of change, intransmutable, invariable, irredeemable, irretrievable, irreversible, irrevocable, lasting, noble, nonreturnable, nonreversible, permanent, reverseless, unalterable, unalterative, unaltered, unchangeable, unchanged, unchanging, undeflectable, undeviating, unmodifiable, unpayable, unremitting, unrestorable, unreturnable, unsusceptible, unvariable, unvarying