final
incommutable
incurable
last
unchangeable
adamant
adamantine
cast-iron
changeless
constant
dour
firm
fixed
flinty
grim
hard
hard-core
immovable
immutable
implacable
impliable
inalterable
inconvertible
indefeasible
inductile
inelastic
inert
inexorable
inextensible
inextensile
inextensional
inflexible
insusceptible of change
intractable
intractile
intransigent
intransmutable
invariable
iron
irreconcilable
irresilient
irretrievable
irreversible
irrevocable
lasting
noble
nonelastic
nonreturnable
nonreversible
nonstretchable
permanent
relentless
ineradicable
instinctive
inviolable
laudable
steadfast
unanimous
unconquerable
undeviating
undying
unfailing
unflinching
unquestionable
unquestioning
unshakable
unshaken
alterable