Flint is a variety of chalcedony (cryptocrystalline quartz) known for its ease of forming blades (including spear tips and arrowheads). It is a fine-grained quartz whose crystals are too small to see.
Flint may be black, gray, brown, and occasionally dark green or even white. It is found in nodules, often with a white coating. The microscopic crystals of flint have a fibrous habit, which results in a notable behavior when chipped: the pieces are sharp, thin splinters called blades (if large) or flakes (if small).
Repeated chipping (knapping) a piece of flint can result in a carefully shaped arrowhead with an extremely sharp edge.
Another use of flint is in a "flintlock", a device where a small piece of flint is struck against a steel plate, releasing sparks to ignite gunpowder or start a fire.
Flint is only one of several chalcedony (cryptocrystalline quartz) varieties. The primary varieties are as follows: