Diamond, especially as used in French or as a fancy English term; a precious gem of crystallized carbon.
From French diamant, from Medieval Latin diamant-, from Latin adamant- 'invincible,' from Greek adamas. The word traveled through Romance languages, retaining Old French form in English when referring to gems decoratively.
Diamant is why 'adamant' (meaning unwilling to change) comes from the same root as diamond—ancient people thought diamonds were 'invincible stones,' and that unbreakable quality metaphorically became our word for stubborn people.
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