How do you tell that a rock is flint?

Answer 1

Flint is a common name for a form of sedimentary rock called chert, formed from microscopic siliceous (essentially quartz) skeletons of radiolarians. As such the hardness of chert is 7, which is slighly harder than glass and considerably harder than steel. If it will scratch glass it is likely to be flint. Flint is often mistaken for limestone or marble, which is much softer and will not scratch glass.

Answer 2

Flint is typically dark gray to black in color, has a smooth, glassy texture, and can sometimes have a conchoidal fracture pattern with sharp edges. One way to confirm if a rock is flint is to perform a flint test by striking it against steel to see if sparks are produced.