Adamantine

adj. hard, tough; unyielding; having sparkle and luster like a diamond

Adamantine is a mineral, often referred to as adamantine spar. It is a silky brown form of corundum. It has a Mohs rating of 9.Adamantine is also used as an adjective to refer to non-metallic, brilliant light reflecting and transmitting properties, known as adamantine luster. Diamond is the best known material to be described as having adamantine lustre, although anglesite, cerussite and corundum in some of its forms are also described in this way.
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Adjective
1. consisting of or having the hardness of adamant
(pertainym) diamond, adamant
2. having the hardness of a diamond
(similar) hard
3. not capable of being swayed or diverted from a course; unsusceptible to persuasion; "he is adamant in his refusal to change his mind"; "Cynthia was inexorable; she would have none of him"- W.Churchill; "an intransigent conservative opposed to every liberal tendancy"
(synonym) adamant, inexorable, intransigent
(similar) inflexible

adj. adamant, unyielding, immovable

adj. adamantine, adamant, hard, unyielding