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]