Adamantine

adamantine [ad-uh-MAN-teen]
adj. impenetrably or unyieldingly hard

[Derives from Latin adamas ("hard"), same origin as diamond]

While adamant is a relatively common word in contemporary English -- "Sen. Boxer was adamant in her opposition to the agriculture appropriations' bill" -- adamantine is seen less often. As adjectives, they are essentially synonymous.
Adamant is generally used to describe attitudes or opinions, but can be applied to physical objects which are "too hard to cut, break, or pierce." Adamantine is more commonly applied to physical objects, situations or personalities which are implacable, inexorable, inflexible, intransigent or obdurate. Yet it can be used interchangeably with adamant in modifying an opinion or an attitude which is unbreakably hard.

In Theodore Rex, Edmund Morris describes Teddy Roosevelt's perception of a picture of Woodrow Wilson in a magazine in 1902, ten years before Roosevelt would lose the presidential election to Wilson:

Pale eyes absolutely lacking in self-doubt, an unfurrowed brow, haughty nostrils, long cruel mouth over a tremendous jaw -- features both intellectual and tough, adamantine in their cold, smooth power: it was the beaky professor who had visited with him in Buffalo the day after his inauguration.