Etymology
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adamant (n.)
Old English aðamans "a very hard stone;" the modern word is a mid-14c. borrowing of Old French adamant "diamond; magnet" or directly from Latin adamantem (nominative adamas) "adamant, hardest iron, steel," also used figuratively, of character, from Greek adamas (genitive adamantos), name of a hypothetical hardest material, noun use of an adjective meaning "unbreakable, inflexible," which was metaphoric of anything unalterable (such as Hades), a word of uncertain origin.

It is perhaps literally "invincible, indomitable," from a- "not" (see a- (3)) + daman "to conquer, to tame," from PIE root *deme- "to constrain, force, break (horses)" (see tame (adj.)). "But semantically, the etymology is rather strange," according to Beekes, who suggests it might be a foreign word altered in Greek by folk etymology, and compares Akkadian (Semitic) adamu.

Applied in antiquity to a metal resembling gold (Plato), white sapphire (Pliny), magnet (Ovid, perhaps through confusion with Latin adamare "to love passionately"), steel, emery stone, and especially diamond, which is a variant of this word. "The name has thus always been of indefinite and fluctuating sense" [Century Dictionary].
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adamant (adj.)

late 14c., "hard, unbreakable," from adamant (n.). Figurative sense of "unshakeable" is by 1670s. Related: Adamantly; adamance.

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adamantine (adj.)
"made of adamant; having the qualities of adamant" (hard, unyielding, unbreakable, inflexible), c. 1200, from Latin adamantinus "hard as steel, inflexible," from Greek adamantinos "hard as adamant," from adamas (genitive adamantos) "unbreakable, inflexible," as a noun, "hardest material" (see adamant (n.)).
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a- (3)
prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).

In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.

Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."
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diamond (n.)

mid-14c., diamaunt, diamond, "extremely hard and refractive precious stone made of pure or nearly pure carbon," from Old French diamant, from Medieval Latin diamantem (nominative diamas), from Vulgar Latin *adiamantem (which was subsequently altered by influence of the many Greek words in dia-), from Latin adamantem (nominative adamans) "the hardest metal," later, "diamond," from Greek adamas (genitive adamantos), name of a hypothetical hardest material, noun use of an adjective meaning "unbreakable, inflexible," a word of uncertain origin (see adamant (n.)).

From early 15c. as "person of great worth" (a sense also in Latin). From late 15c. as "geometric figure of four equal straight lines forming two acute and two obtuse angles." From 1590s as "playing-card stamped with one or more red diamonds." In baseball, "square space enclosed within the four bases," is American English, by 1875. As an adjective "resembling, consisting of, or set with diamonds," from 1550s.

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magnet (n.)

"variety of magnetite characterized by its power of attracting iron and steel," mid-15c. (earlier magnes, late 14c.), from Old French magnete "magnetite, magnet, lodestone," and directly from Latin magnetum (nominative magnes) "lodestone," from Greek ho Magnes lithos "the Magnesian stone," from Magnesia (see magnesia), region in Thessaly where magnetized ore was obtained. Figurative sense of "something which attracts" is from 1650s.

It has spread from Latin to most Western European languages (German and Danish magnet, Dutch magneet, Italian, Spanish, Portuguese magnete), but it was superseded in French by aimant (from Latin adamas; see adamant (n.)). Italian calamita "magnet" (13c.), French calamite (by 16c., said to be from Italian), Spanish caramida (15c., probably from Italian) apparently is from Latin calamus "reed, stalk or straw of wheat" (see shawm) "the needle being inserted in a stalk or piece of cork so as to float on water" [Donkin]. Chick magnet attested from 1989.

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