adamantine \ad-uh-MAN-teen\ adjective
1 : made of or having the quality of adamant
*2 : rigidly firm : unyielding
3 : resembling the diamond in hardness or luster

Example sentence:
Jay was adamantine in his refusal to help with the party, insisting he had to be somewhere else that night

Did you know?
The Greek and Latin word for the hardest imaginable substance, whether applied to a legendary stone or an actual substance, such as diamond, was "adamas." Latin poets used the term figuratively for things lasting, firm, or unbending, and the adjective "adamantinus" was used in similar contexts. The English noun "adamant" (meaning "an unbreakable or extremely hard substance"), as well as the adjective "adamant" (meaning "inflexible" or "unyielding"), came from "adamas." "Adamantine," which has such figurative uses as "rigid," "firm," and "unyielding," came from "adamantinus." "Adamas" is actually the source of "diamond" as well. "Diamas," the Latin term for diamond, was an alteration of "adamas."

*Indicates the sense illustrated in the example sentence.