Adamant /(ăd"ȧ*mănt)/

Ad·a·mant

Adamant

n.
  1. A stone imagined by some to be of impenetrable hardness; a name given to the diamond and other substances of extreme hardness; but in modern mineralogy it has no technical signification. It is now a rhetorical or poetical name for the embodiment of impenetrable hardness.
    Opposed the rocky orb Of tenfold adamant, his ample shield.
  2. Lodestone; magnet. [Obs.]
    As true to thee as steel to adamant.