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.
- Impervious to pleas, appeals, or reason; stubbornly unyielding. See Synonyms at inflexible.
- A stone once believed to be impenetrable in its hardness.
- An extremely hard substance.