The Collaborative International Dictionary of English v.0.48:

Inconvertible \In`con*vert"i*ble\, a. [L. inconvertibilis: cf.
   F. inconvertible. See In- not, and Convertible.]
   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. --Walsh.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconvertible
    adj 1: used especially of currencies; incapable of being
           exchanged for or replaced by another currency of equal
           value [syn: inconvertible, unconvertible,
           unexchangeable] [ant: convertible, exchangeable]
    2: not capable of being changed into something else; "the
       alchemists were unable to accept the inconvertible nature of
       elemental metals" [syn: inconvertible, untransmutable]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "inconvertible":
   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