Flint (or flintstone) is a hard, sedimentary cryptocrystalline silicate form of the mineral quartz, categorized as a variety of chalcedony and broadly part of the mineral group known as silicas. Flint is usually dark-grey, blue, black, or deep brown in colour, and often has a glassy appearance. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones.
Flint was one of the most commonly used materials for the manufacture of stone tools during the Stone Age, as it splits into thin, sharp splinters called flakes or blades (depending in the shape) when struck by another hard object (such as a hammerstone made of another material). This process is referred to as knapping.
Rough flint arrowheads are around 2" long.