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]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "adamant":
   adamantine, at a standstill, cast-iron, dour, firm, flintlike,
   flinty, frozen, granitelike, granitic, grim, hard, hard-core,
   immobile, immotile, immotive, immovable, immutable, implacable,
   impliable, inductile, inelastic, inexorable, inextensible,
   inextensile, inextensional, inflexible, intractable, intractile,
   intransigent, iron, irreconcilable, irremovable, irresilient,
   lithic, marblelike, nonelastic, nonstretchable, obdurate, pat,
   petrified, petrogenic, relentless, rigid, rigorous, rock,
   rock-ribbed, slaty, standpat, stationary, steely, stern, stiff,
   stone, stubborn, unaffected, unalterable, unbending, unchangeable,
   uncompromising, unextendible, unextensible, unflexible, ungiving,
   unlimber, unmalleable, unmovable, unmoved, unmoving, unpliable,
   unpliant, unrelenting, unswayable, untractable, unyielding



Easton's 1897 Bible Dictionary:

Adamant
   (Heb. shamir), Ezek. 3:9. The Greek word adamas means diamond.
   This stone is not referred to, but corundum or some kind of hard
   steel. It is an emblem of firmness in resisting adversaries of
   the truth (Zech. 7:12), and of hard-heartedness against the
   truth (Jer. 17:1).


The Devil's Dictionary (1881-1906):

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