Flint is a very hard and durable variety of quartz. It has been used by humans for thousands of years to make tools and build walls, among many other things. Several major sources can be found in the Northern Hemisphere, and the rock has a number of modern industrial uses. In addition to being included inside many things, flint can also be purchased in raw form, often from hobbyist shops and at rock shows.
Officially, flint is classified as a chalcedony. A chalcedony is a cryptocrystalline form of silica, and a number of minerals including agate and carnelian are also classified as chalcedonies. The term "cryptocrystalline" refers to the form which the crystals of silicate in the rock take. Although the rock is technically crystalline, the individual crystals are so small that they can only be distinguished with the aid of a microscope, unlike larger crystalline quartzes such as amethyst.
Like many rocks, flint is a sedimentary rock, formed over a series of centuries from an assortment of compressed sediments. It can range in color from almost white to black, although it tends to be primarily gray. The extreme hardness of this rock has made it a popular instrument for an assortment of tasks, since it does not readily break down. Early humans used it to make tools such as axes and arrowheads.
Flint also has another unique property that many people are aware of. When struck against steel, fit lint will generate a spark by breaking off a small piece of the steel and heating it, causing the steel to ignite, feeding on the oxygen in the air. Although making a fire with flint and steel can be challenging, it is possible, and the rock is also included in devices like lighters, where it combines with steel above a robust fuel source to make a flame.
There are only a few hazards to humans posed by flint because it is not composed of toxic substances. The edges of broken pieces can be razor sharp, so caution should be used when handling this rock. In areas where flint occurs naturally, a fall onto broken pieces can be rather painful, and the rock has been known to slash through bicycle tires and lightweight pants, as well. Flint is especially common around deposits of chalk, so people should be cautious about going barefoot or participating in rough play in regions of the world with known chalk deposits.