Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. 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 colour, and often has a glassy or waxy appearance. A thin layer on the outside of the nodules is usually different in colour, typically white, and rough in texture. Flint was used for the manufacture of flint tools, as it splits into thin, sharp splinters called flakes or blades (depending on the shape) when struck by another hard object (such as a hammerstone made of another material). This process is referred to as knapping.
Flint can also be used to start a fire; Ayla discovered this technique in The Valley of Horses. After a mistake in making flint weapons, Ayla recognizes that sparks from striking a 'firestone' of iron pyrite with a piece of flint can start a fire.