The Collaborative International Dictionary of English v.0.48:

Flinty \Flint"y\, a. [Compar. Flintier; superl. Flintiest.]
   Consisting of, composed of, abounding in, or resembling,
   flint; as, a flinty rock; flinty ground; a flinty heart.
   [1913 Webster]

   Flinty rock, or Flinty state, a siliceous slate; --
      basanite is here included. See Basanite.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flinty
    adj 1: containing flint
    2: showing unfeeling resistance to tender feelings; "his flinty
       gaze"; "the child's misery would move even the most obdurate
       heart" [syn: flinty, flint, granitic, obdurate,
       stony]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "flinty":
   Philistine, adamant, adamantine, bony, bowelless, callous,
   calloused, case-hardened, cast-iron, cement, cemental, concrete,
   constant, corneous, cruel, dense, diamondlike, dog-eat-dog, dour,
   dure, firm, fixed, flintlike, granitelike, granitic, grim, hard,
   hard as nails, hard-core, hardened, hardhearted, harsh, heartless,
   horny, immovable, immutable, impervious, implacable, impliable,
   inclement, indurated, inelastic, inexorable, inflexible,
   insensitive, intransigent, inured, iron, iron-hard, ironlike,
   irreconcilable, lapideous, lithic, lithoid, lithoidal, loyal,
   marble, marblelike, merciless, obdurate, osseous, pachydermatous,
   petrified, petrogenic, pitiless, proof against, relentless,
   remorseless, resistant, resistive, rigid, rigorous, rock,
   rock-ribbed, rocklike, rocky, ruthless, set, settled, slaty, solid,
   sot, staunch, steadfast, steady, steeled against, steellike,
   steely, stern, stiff, stone, stonelike, stony, thick-skinned,
   tough, true, unaffected, unalterable, unbending, unchangeable,
   uncompassionate, uncompassioned, uncompromising, undeflectable,
   unfeeling, unflappable, unforgiving, ungiving, unmerciful, unmoved,
   unpitiful, unpitying, unrelenting, unremorseful, unshaken,
   unswerving, unsympathetic, unsympathizing, unyielding,
   without mercy