The Collaborative International Dictionary of English v.0.48:

Steely \Steel"y\ (-[y^]), a.
   1. Made of steel; consisting of steel. "The steely point of
      Clifford's lance." --Shak.
      [1913 Webster]

            Around his shop the steely sparkles flew. --Gay.
      [1913 Webster]

   2. Resembling steel; hard; firm; having the color of steel.
      "His hair was steely gray." --The Century.
      [1913 Webster]

            She would unarm her noble heart of that steely
            resistance against the sweet blows of love. --Sir P.
                                                  Sidney.
      [1913 Webster]

   Steely iron, a compound of iron containing less than one
      half of one per cent of carbon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

steely
    adj 1: resembling steel as in hardness; "steely eyes"; "steely
           nerves like those of a steeplejack"
    2: resembling steel in hardness

Moby Thesaurus II by Grady Ward, 1.0:

219 Moby Thesaurus words for "steely":
   Philistine, Quaker-colored, acier, adamant, adamantine, ashen,
   ashy, aureate, beefy, bony, bouncing, brass, brassy, brazen,
   bronze, bronzy, callous, calloused, canescent, case-hardened,
   cast-iron, cement, cemental, cinereous, cinerous, concrete,
   constant, copper, coppery, corneous, cupreous, cuprous, dapple,
   dapple-gray, dappled, dappled-gray, dense, diamondlike, dingy,
   dismal, doughty, dour, dove-colored, dove-gray, dreary, dull, dure,
   dusty, ferrous, ferruginous, firm, fixed, flintlike, flinty,
   forceful, forcible, forcy, full-blooded, full-strength, gilt,
   glaucescent, glaucous, gold, gold-filled, gold-plated, golden,
   granitelike, granitic, gray, gray-black, gray-brown, gray-colored,
   gray-drab, gray-green, gray-spotted, gray-toned, gray-white,
   grayed, grayish, grey, grim, griseous, grizzle, grizzled, grizzly,
   gutsy, gutty, hale, hard, hard as nails, hard-core, hardened,
   hardhearted, hardy, hearty, hefty, horny, husky, immovable,
   immutable, impervious, implacable, impliable, indurate, indurated,
   inelastic, inexorable, inflexible, insensitive, intransigent,
   inured, iron, iron-gray, iron-hard, ironlike, irreconcilable,
   lapideous, lead, lead-gray, leaden, lithoid, lithoidal, livid,
   loyal, lusty, marble, marblelike, mercurial, mercurous, mighty,
   mouse-colored, mouse-gray, mousy, nervy, nickel, nickelic,
   nickeline, obdurate, obstinate, osseous, pachydermatous, pearl,
   pearl-gray, pearly, pewter, pewtery, potent, powerful,
   proof against, puissant, quicksilver, red-blooded, relentless,
   resistant, resistive, rigid, rigorous, robust, robustious,
   rock-ribbed, rocklike, rocky, rugged, sad, set, settled, silver,
   silver-gray, silver-plated, silvered, silvery, slate-colored,
   slaty, smoke-gray, smoky, sober, solid, somber, sot, stalwart,
   staunch, steadfast, steady, steel, steel-gray, steeled against,
   steellike, stern, stiff, stone-colored, stonelike, stony, stout,
   strapping, strong, strong as brandy, strong as strong,
   strong-willed, sturdy, taupe, thick-skinned, tin, tinny, tough,
   true, unaffected, unalterable, unbending, unchangeable,
   uncompromising, undeflectable, unflappable, ungiving, unmoved,
   unrelenting, unshaken, unswerving, unyielding, vigorous