adamantine

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."