Adamant

utterly unyielding in att-tude or opinion in spite of all appeals, urgings, etc.
too hard to cut, break, or pierce.
any impenetrably or unyieldingly hard substance.
a legendary stone of impenetrable hardness, formerly sometimes identified with the diamond.
contemporary examples

i was adamant that she was too young and took the newspaper to my parents for further authority.
spilling my family’s secrets frances osborne may 20, 2010

the guardian is adamant that neither davies nor hill has paid any police officer for any information.
the appalling probe on the guardian alex m-ssie september 16, 2011

blankfein & co. was adamant that there was no conflict at all.
the hearing from h-ll tunku varadarajan april 26, 2010

she is adamant about the difference between online dating and her own bespoke matches.
anna gristina, the accused mommy madam, and her matchmaking defense tracy quan june 24, 2012

despite this constant pressure, the justices—the older ones, at least—are adamant.
supreme court releases same-day audio, but forget tv nick summers march 29, 2012

historical examples

iron and adamant are not stronger than these arguments; nor can any one attempt an answer without exposing his feebleness.
charles sumner; his complete works, volume 6 (of 20) charles sumner

virtue is an adamant that is sacred and secure from all their efforts.
imogen william g-dwin

but who would force the soul, tilts with a straw / against a champion cased in adamant.
dictionary of quotations from ancient and modern, english and foreign sources james wood

“oh, do let’s stay till it’s all done,” she urged, but bruce and elinor were adamant.
miss pat at school pemberton ginther

acts of parliament are venerable; but if they correspond not with the writing on the ‘adamant tablet,’ what are they?
past and present thomas carlyle

adjective
unshakable in purpose, determination, or opinion; unyielding
a less common word for adamantine (sense 1)
noun
any extremely hard or apparently unbreakable substance
a legendary stone said to be impenetrable, often identified with the diamond or loadstone
adj.

late 14c., “hard, unbreakable,” from adamant (n.). figurative sense of “unshakeable” first recorded 1670s. related: adamantly; adamance.
n.

mid-14c., from old french adamant and directly from latin adamantem (nominative adamas) “adamant, hardest iron, steel,” also figuratively, of character, from greek adamas (genitive adamantos) “unbreakable, inflexible” metaphoric of anything unalterable, also the name of a hypothetical hardest material, perhaps literally “invincible,” from a- “not” + daman “to conquer, to tame” (see tame (adj.)), or else a word of foreign origin altered to conform to greek.

applied in antiquity to white sapphire, magnet (perhaps via confusion with latin adamare “to love p-ssionately”), steel, emery stone, and especially diamond (see diamond). the word was in old english as aðamans “a very hard stone.”

(heb. shamir), ezek. 3:9. the greek word adamas means diamond. this stone is not referred to, but corundum or some kind of hard steel. it is an emblem of firmness in resisting adversaries of the truth (zech. 7:12), and of hard-heartedness against the truth (jer. 17:1).