adamantine

This term carries a high degree of formality and is frequently found in literary, poetic, or archaic contexts. It evokes the image of a diamond, suggesting not just hardness, but a crystalline, shimmering quality of strength. When used to describe physical objects, it implies an absolute resistance to damage or wear.

In a psychological or emotional sense, the word describes a resolve that is impervious to persuasion or fatigue. It is more intense than "firm" or "steadfast," suggesting a rigidity that is nearly supernatural or absolute. Because of its intensity, using it in casual conversation may sound overly dramatic or pretentious.