inconvertible

Adjective

• not capable of being changed into something else
  • the alchemists were unable to accept the inconvertible nature of elemental metals
    

• used especially of currencies; incapable of being exchanged for or replaced by another currency of equal value

Related words

• Synonyms: 
  untransmutable , unconvertible , unexchangeable
• Antonyms: 
  convertible , exchangeable