The Collaborative International Dictionary of English v.0.48:

Indestructible \In`de*struc"ti*ble\, a. [Pref. in- not +
   destructible: cf. F. indestructible.]
   Not destructible; incapable of decomposition or of being
   destroyed; as, the Russian harvesters had primitive
   electronics, but they were built so sturdily they were almost
   indestructible. -- In`de*struc"ti*ble*ness, n. --
   In`de*struc"ti*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indestructible
    adj 1: not easily destroyed [ant: destructible]
    2: very long lasting; "less durable rocks were gradually worn
       away to form valleys"; "the perdurable granite of the ancient
       Appalachian spine of the continent" [syn: durable,
       indestructible, perdurable, undestroyable]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "indestructible":
   ageless, ceaseless, changeless, coeternal, constant, continual,
   continuous, dateless, deathless, durable, endless, enduring,
   eternal, eterne, ever-being, ever-durable, ever-during,
   everlasting, everliving, fixed, immemorial, immortal, immutable,
   imperishable, impregnable, incessant, incorruptible, indelible,
   ineffaceable, ineradicable, inexpugnable, inextinguishable,
   infinite, interminable, invincible, invulnerable, irrefragable,
   lasting, long-lasting, never-ceasing, never-ending, nonperishable,
   nonstop, nonterminating, nonterminous, olamic, perdurable,
   perennial, permanent, perpetual, quenchless, sempiternal, steady,
   timeless, unalterable, unbreakable, unceasing, unchangeable,
   unchanging, undampable, undestroyable, undying, unending,
   unintermitting, uninterrupted, unquenchable, unremitting,
   without end