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.(noun)
source: webster1913
adjective satellite (s)
impervious to pleas, persuasion, requests, reason(adj.all)
Example: He is adamant in his refusal to change his mind.