Flint

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.

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

Anything extremely hard, unimpressible, and unyielding, like flint.

a hard kind of stone; a form of silica more opaque than chalcedony