The Collaborative International Dictionary of English v.0.48:

Adamant \Ad"a*mant\ ([a^]d"[.a]*m[a^]nt), n. [OE. adamaunt,
   adamant, diamond, magnet, OF. adamant, L. adamas, adamantis,
   the hardest metal, fr. Gr. 'ada`mas, -antos; 'a priv. +
   dama^,n to tame, subdue. In OE., from confusion with L.
   adamare to love, be attached to, the word meant also magnet,
   as in OF. and LL. See Diamond, Tame.]
   1. A stone imagined by some to be of impenetrable hardness; a
      name given to the diamond and other substances of extreme
      hardness; but in modern mineralogy it has no technical
      signification. It is now a rhetorical or poetical name for
      the embodiment of impenetrable hardness.
      [1913 Webster]

            Opposed the rocky orb
            Of tenfold adamant, his ample shield. --Milton.
      [1913 Webster]

   2. Lodestone; magnet. [Obs.] "A great adamant of
      acquaintance." --Bacon.
      [1913 Webster]

            As true to thee as steel to adamant.  --Greene.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adamant
    adj 1: impervious to pleas, persuasion, requests, reason; "he is
           adamant in his refusal to change his mind"; "Cynthia was
           inexorable; she would have none of him"- W.Churchill; "an
           intransigent conservative opposed to every liberal
           tendency" [syn: adamant, adamantine, inexorable,
           intransigent]
    n 1: very hard native crystalline carbon valued as a gem [syn:
         diamond, adamant]

The Devil's Dictionary (1881-1906):

ADAMANT, n.  A mineral frequently found beneath a corset.  Soluble in
solicitate of gold.