adamant
adamantine
immovable
immutable
inelastic
inextensible
inextensile
inextensional
inflexible
intractable
intractile
irresilient
nonelastic
nonstretchable
stubborn
tough
unalterable
unbending
unextendible
unextensible
unflexible
ungiving
unlimber
unmalleable
unpliable
unpliant
untractable
unyielding