(1):

(n.) A piece of flint for striking fire; - formerly much used, esp. in the hammers of gun locks.

(2):

(n.) Anything extremely hard, unimpressible, and unyielding, like flint.

(3):

(n.) A massive, somewhat impure variety of quartz, in color usually of a gray to brown or nearly black, breaking with a conchoidal fracture and sharp edge. It is very hard, and strikes fire with steel.