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Adjective: adamantine  ,a-du'man,teen or ,a-du'man,tIn
  1. Consisting of or having the hardness of adamant
  2. Having the hardness of a diamond
  3. Impervious to pleas, persuasion, requests, reason
    "he is adamantine in his refusal to change his mind";
    - adamant, inexorable, intransigent

See also: hard, inflexible

Encyclopedia: Adamantine