Chert (/tʃɜːrt/) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz,[1] the mineral form of silicon dioxide (SiO2).

[2] Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

[4][12] In petrology, the term "chert" refers generally to all chemically precipitated sedimentary rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous silica.

[17] Chert is found in settings as diverse as hot spring deposits (siliceous sinter), banded iron formation (jaspilite),[8] or alkaline lakes.

[21] Bedded chert is most often found in association with turbidites, deep water limestone, submarine volcanic rock,[23] ophiolites, and mélanges on active margins of tectonic plates.

)[25] Such impurities as are present include authigenic pyrite and hematite, formed in the sediments after they were deposited, in addition to traces of detrital minerals.

Once the organisms die, their skeletons will quickly dissolve unless they accumulate on the ocean bottom and are buried, forming siliceous ooze that is 30% to 60% silica.

Thus, bedded cherts are typically composed mostly of fossil remains of organisms that secrete silica skeletons, which are usually altered by solution and recrystallization.

[26] The skeletons of these organisms are composed of opal-A, an amorphous form of silica, lacking long-range crystal structure.

This is gradually transformed to opal-CT, a microcrystalline form of silica composed mostly of bladed crystals of cristobalite and tridymite.

[29] There is evidence that the variety of chert called porcelainite, which is characterized by a high content of opal-CT, recrystallizes at very shallow depths.

Beds of marine diatomaceous chert comprising strata several hundred meters thick have been reported from sedimentary sequences such as the Miocene Monterey Formation of California and occur in rocks as old as the Cretaceous.

They are found in association with glauconite-rich sandstone, black shale, clay-rich limestone, phosphorites, and other nonvolcanic rocks typical of water a few hundred meters deep.

Their origin is uncertain, but they may form from fossil remains that are completely dissolved in fluids that then migrate to precipitate their silica load in a nearby bed.

The nodules are most typically along bedding planes or stylolite (dissolution) surfaces, where fossil organisms tended to accumulate and provided a source of dissolved silica, but they are sometimes found cutting across bedding surfaces, where the chert fills fossil burrows, fluid escape structures, or fractures.

Most chert nodules have textures suggesting they were formed by diagenetic replacement, where silica was deposited in place of calcium carbonate or clay minerals.

[40][41] Nonmarine cherts may form in saline alkaline lakes as thin lenses or nodules showing sedimentary structures suggestive of evaporite origin.

Episodes of runoff of fresh water into the lakes lowers the pH and precipitates the unusual sodium silicate minerals magadiite or kenyaite, After burial and diagenesis, these are altered to Magadi-type chert.

[43] Chert may also form from replacement of calcrete in fossil soils (paleosols) by silica dissolved from overlying volcanic ash beds.

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

Like obsidian, as well as some rhyolites, felsites, quartzites, and other tool stones used in lithic reduction, chert fractures in a Hertzian cone when struck with sufficient force.