Like other sedimentary rocks, chert starts with particles accumulating. In this case, the setting is in bodies of water and the particles are the skeletons (called tests) of planktonmicroscopic creatures that spend their lives floating in the water column. Plankton secrete their tests using one of two substances that are dissolved in water: calcium carbonate or silica. When the organisms die, their tests sink to the bottom and accumulate in a growing blanket of microscopic sediment called ooze.
Ooze is usually a mixture of plankton tests and extremely fine grained clay minerals. A clay ooze, of course, eventually becomes claystone. An ooze that is primarily calcium carbonate (aragonite or calcite), a calcareous ooze, typically turns into a rock of the limestone group. The creation of chert requires a siliceous ooze. The composition of ooze depends on details of geography: ocean currents, the availability of nutrients in the water, world climate, depth in the ocean and much more.
Siliceous ooze is mostly made of the tests of diatoms (one-celled algae) and radiolarians (one-celled "animals" or protists). These organisms build their tests of completely uncrystallized (amorphous) silica. Other minor sources of silica skeletons include the particles made by sponges (spicules) and land plants (phytoliths). Siliceous ooze tends to form in cold, deep water because calcareous tests dissolve in those conditions.
Chert Formation and Precursors
Siliceous ooze turns to chert by going through a slow transformation unlike that of most other rocks. The lithification and diagenesis of chert is an elaborate process we still have much to learn about.
In some settings, siliceous ooze is pure enough to lithify into a lightweight, minimally processed rock, called diatomite if composed of diatoms or radiolarite if made of radiolarians. But usually things don't stop there.
The amorphous silica of a plankton test is not stable outside the living things that make it. It seeks to crystallize, and as ooze is buried to depths greater than 100 meters or so, the silica begins to mobilize with the modest rise in pressure and temperature. There is plenty of pore space and water for this to happen, and plenty of chemical energy being released by crystallization as well as by the breakdown of organic matter in the ooze.
The first product of this activity is a hydrated silica (opal) called opal-CT because it resembles cristobalite (C) and tridymite (T) in X-ray studies. In those minerals, silicon and oxygen atoms stack together with water molecules in a different arrangement than that of quartz. A less-processed version of opal-CT is what makes up common opal. A more-processed version of opal-CT is often called opal-C because in X-rays it looks more like cristobalite. The rock composed of lithified opal-CT or opal-C is porcellanite.
More diagenesis causes the silica to lose most of its water as it fills pore space in the siliceous sediment. This activity converts the silica into true quartz, in microcrystalline or cryptocrystalline form, also known as the mineral chalcedony. This rock is now true chert.
Chert Attributes and Signs
Chert is as hard as crystalline quartz, hardness 7 in the Mohs scalemaybe a bit softer, 6.5, if it still has some hydrated silica in it. Beyond simply being hard, chert is a tough rock. It stands above the landscape in outcrops that resist erosion. Oil-well drillers dread it because it's so hard to penetrate.
Chert has a curvy conchoidal fracture that is smoother and less splintery than the conchoidal fracture of pure quartz; ancient tool makers favored it, and high-quality rock was a trade item between tribes.
Unlike quartz, chert is never transparent and not always translucent. It has a waxy or resinous luster unlike the glassy luster of quartz. Whereas sandstone is made of quartz grains, chert is made of quartz stuff.
The colors of chert range from white through red and brown to black, depending on how much clay or organic matter it contains. It often has some sign of its sedimentary origin, such as bedding and other sedimentary structures or microfossils. They may be abundant enough for a chert to get a special name, as in the red radiolarian chert carried to land by plate tectonics from the central ocean floor.
Chert is a quite general term for noncrystalline siliceous rocks, and some subtypes have their own names and stories.
In mixed calcareous and siliceous sediments, the carbonate and the silica tend to segregate. Chalk beds, the calcareous equivalent of diatomites, may grow lumpy nodules of chert of the type called flint. (Similarly, thick chert beds may grow nodules and pods of limerocklimestone or dolomite rock.) Flint is commonly dark and gray, and more lustrous than typical chert.
Agate and jasper are cherts that form outside the deep-sea setting; instead they occur where fractures allowed silica-rich solutions to enter and deposit chalcedony. Agate is pure and translucent whereas jasper is opaque. Both stones commonly have reddish colors from the presence of iron oxide minerals. The peculiar ancient banded iron formations consist of thin layers of interbedded chert and solid hematite.
Some important fossil localities are in chert. The Rhynie Cherts, in Scotland, contain remains of the oldest land ecosystem from nearly 400 million years ago early in the Devonian Period. And the Gunflint Chert, a unit of banded iron formation in western Ontario, is famous for its fossil microbes, dating from Early Proterozoic time some 2 billion years ago.
What Chert Means
As we have seen, chert is primarily a seafloor rock that forms at shallow depths of burial. Because it's made almost entirely of the bodies of plankton, chert is very often rich in organic matter and typically an important source rock for petroleum. The diagenetic process that turns ooze into stone helps the organic matter escape while the mineral matter consolidates.
In the cycle of plate tectonics, most of the material on the world's seafloor ends up recycled into the deep Earth through the process of subduction. Therefore the silica in chert must be an important ingredient in arc volcanism, ore formation, silica enrichment of magma, and all the other processes that maintain the continents against erosion. The quartz found everywhere in veins, in granites and ultimately in sandstones and beaches has surely been recycled from chert at least once in Earth's history.