"inconvertible" in the adjective sense
1. inconvertible, unconvertible, unexchangeable
used especially of currencies incapable of being exchanged for or replaced by another currency of equal value
2. inconvertible, untransmutable
not capable of being changed into something else
"the alchemists were unable to accept the inconvertible nature of elemental metals"
Source: WordNet® (An amazing lexical database of English)
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