Chert is a fine-grained silica-rich cryptocrystalline sedimentary rock that may contain small fossils. It varies greatly in color from white to black, but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements present in the rock, and both red and green are most often related to traces of iron (in its oxidized and reduced forms respectively). It outcrops as nodules in limestone, chalk, and dolostone formations as a replacement mineral, as well as in thin beds when it is a primary deposit, and is generally considered to be less attractive and more common than flint, although the two materials are closely related. In geological terms flint and chert are the same, with the term flint referring to chert found in chalk.

Chert and Precambrian fossils

The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallisation and metamorphism has made it an ideal rock for preservation of early life forms.

For example:

– The 3.2 billion year old chert of the Fig Tree Formation in the Barbeton Mountains between Swaziland and South Africa preserved non-colonial unicellular bacterial-like fossils.

– The Gunflint Chert of western Ontario (1.9 to 2.3 By) preserves not only bacteria and cyanobacteria but also organisms believed to be ammonia-consuming and some that resemble green algae and fungus-like organisms.

– The Apex Chert (3.4 By) of the Pilbara Craton, Australia preserved eleven taxa of prokaryotes.

– The Devonian Rhynie Chert (400 My) of Scotland has the oldest remains of land flora, and the preservation is so perfect that it allows cellular studies of the fossils.

Chert and flint: archaeological and historical uses

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. This results in conchoidal fractures, a characteristic of all minerals with no cleavage planes. 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 flakes, 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. 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, discharging the firearm.

In some areas chert is ubiquitous as stream gravel and fieldstone and is used as construction material and road surfacing.