glassy rock that breaks to a sharp edge


Chert is a stone found as nodules in soft chalky rock, often river beds. Often thought of as something separate, flint is a shiny type of chert. There are two main uses for it.

It can be knapped (worked and shaped) with tools to make sharp implements. These can be simple flakes or they can be sophisticated double-edged blades, shaped in a way that optimizes both strength and sharpness.

Cherts are also important as a source of sparks, for making fires. This is generally done at dawn or dusk, as flint sparks are not so bright as to be easily seen by daylight.


Although we usually think of it as inorganic, chert is a mineral of organic origin. It begins as a pile of glass-containing diatom shells settling on the floor of a lake or ocean. Over time, pressure of material accumulated above it presses it hard. Glass is actually a liquid and the tiny fragments eventually fuse, even at cool temperatures.

Some paleolithic people discover “heat tempering”: the practice of baking flint nodules. This improves its durability and shattering properties. As heated rocks may contain pockets of gas, exploding is another possibility, so the process can be risky. Presumably the stones to be treated would be placed underneath a bonfire, and gathered out of the ashes later.