flint

n. 1 a hard grey siliceous stone. B piece of this, esp. As a primitive tool or weapon. 2 piece of hard alloy used to give a spark. 3 anything hard and unyielding.  flinty adj. (-ier, -iest). [old english]

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.