adamantine

Made of adamant (“an unspecified mineral or rock of virtually impenetrable hardness”).

Incapable of being broken, dissolved, or penetrated; impenetrable, unbreakable.

Difficult to defeat or prevail over; unshakable, unyielding.

Of a person: refusing to change one's mind; obstinate, stubborn.

Having the quality of attracting or drawing; attractive, magnetic.

Like diamond in lustre; bright, lustrous, shiny; also, of a lustre: like that of a mineral with a high refractive index such as diamond.