Flint (or flintstone) is a hard, sedimentary cryptocrystalline form of the mineral quartz. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in color, and often has a glassy or waxy appearance
Flint a.k.a. silica or quartz is a prime ingredient in glaze. It is the “glass” maker. The ratio between the alumina and silica determine the texture of the glaze and the ratio between the fluxes and silica determine the melting point.