ChertChert is a finegrained cryptocrystalline sedimentary rock that often contains small fossils. It varies in color from white to black, but most often manifests as gray, brown, and grayish brown. It outcrops as nodules in limestone and chalk formations, and is generally considered to be less attractive and more common than flint, although the two materials are closely related.
In prehistoric times, chert was often used as a source material for stone tools. Like flint, obsidian, and chalcedony, as well as some rhyolites, felsites, quartzites and a few other tool stones used in lithic reduction, chert fractures in a Hertzian cone when struck with sufficient force. In this kind of fracture, a cone of force propagates through the material from the point of impact, eventually removing a full or partial cone; this result is familiar to anyone who has seen what happens to a plate-glass window when struck by a small object, such as an airgun projectile. The partial Hertzian cones produced during lithic reduction are called flakess, and exhibit features characteristic of this sort of breakage, including striking platforms, bulbs of force, and occasionally eraillures, which are small secondary flakes detached from the flake's bulb of force.
When a chert stone is struck against steel, sparks result. This makes it an excellent tool for starting fires, and both flint and chert were used in various types of fire-starting tools, such as tinderboxes, throughout history. This practice continues up until today for cigarette lighters and certain specialized tools like the igniters used by modern welders to ignite their torches. A primary historic use of chert was as flints for flintlock firearms, in which flint or chert striking a metal plate produces a spark that ignites a small reservoir containing black powder, igniting it and discharging the firearm. Modern uses of chert are generally limited to construction activities.