Flint

Flint, occasionally flintstone (SiO2 ), is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start fires. Flint occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey or black, green, white, or brown in colour, and has a glassy or waxy appearance. A thin, oxidised layer o...