flint

nouns

a hard kind of stone; a form of silica more opaque than chalcedony
is derived from
is a type of (narrower)
a white or colorless vitreous insoluble solid (SiO2); various forms occur widely in the earth's crust as quartz or cristobalite or tridymite or lechatelierite

adjectives

showing unfeeling resistance to tender feelings
"his flinty gaze"; "the child's misery would move even the most obdurate heart"