inalienable
unalienable
unbreakable
chip-proof
coherent
cohesive
firm
firm as Gibraltar
fractureproof
impartible
indiscerptible
indissoluble
indissolvable
indivisible
infusible
inseparable
insoluble
made of iron
nonbreakable
rigid
shatterproof
solid
sound
stable
staunch
stout
sturdy
undividable
unified
unshatterable