If you stubbornly refuse to change your mind about something, you are adamant about it.

This word's story begins in ancient Greece, where philosophers spoke about a legendary unbreakable stone or metal they called adamas (literally, "invincible"). In English, people began to use the word to refer to something that cannot be altered, and then in the twentieth century — after adamant had been in English for about a thousand years — it came to be used as an adjective to mean "unyielding as stone." If you're adamant about something, no amount of persuasion is going to convince you otherwise.

Definitions of adamant

n very hard native crystalline carbon valued as a gem

black diamond, carbonado
an inferior dark diamond used in industry for drilling and polishing
Type of:
C, atomic number 6, carbon
an abundant nonmetallic tetravalent element occurring in three allotropic forms: amorphous carbon and graphite and diamond; occurs in all organic compounds
transparent gem
a gemstone having the property of transmitting light without serious diffusion

adj impervious to pleas, persuasion, requests, reason

“he is adamant in his refusal to change his mind”
adamantine, inexorable, intransigent
incapable of change