(Adjective) That cannot be easily overcome or defeated; rigid, unyielding; unwavering, immovable; (now especially) refusing to be persuaded or to change one’s mind.

In Context

“Contrary to popular impression, the real art of colonial government was not an adamantine rigidity but a machiavellian flexibility in the perpetual search for collaborators.”

Robert Holland, Editor, Emergencies and Disorder in the European Empires after 1945, 1994.