Chert Chert FAQ

Chert Chert FAQ


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Home » Rocks » Sedimentary Rocks » Chert


What Is Chert?   How Does It Form?   What Is It Used For?

Article by: Hobart M. King , Ph.D., RPG


Chert: This specimen of gray chert is about two inches (five centimeters) across. It breaks with a smooth conchoidal fracture. Edges of the piece have sharp edges as a result of the conchoidal fracture.

What is Chert?

Chert is a sedimentary rock composed of microcrystalline or cryptocrystalline quartz , the mineral form of silicon dioxide (SiO2). It occurs as nodules, concretionary masses, and as layered deposits.

Chert breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. “Chert” and ” flint ” are names used for the same material. Both are varieties of chalcedony.

Is this Rock Flint? Chert? or Jasper?

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Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination.

How Does Chert Form?

Chert can form when microcrystals of silicon dioxide grow within soft sediments that will become limestone or chalk . In these sediments, enormous numbers of silicon dioxide microcrystals grow into irregularly-shaped nodules or concretions when dissolved silica is transported to the formation site by the movement of groundwater.

If the nodules or concretions are numerous, they can grow large enough to merge with one another to form a nearly continuous layer of chert within the sediment mass. Chert formed in this manner is a chemical sedimentary rock .

Diatoms are microscopic, single-celled algae that live in marine or fresh water.
They produce hard parts made of silicon dioxide. NASA Image.

Some of the silicon dioxide in chert is thought to have a biological origin. In some parts of the ocean and in shallow seas, large numbers of diatoms and radiolarians live in the water. These organisms have a glassy silica skeleton. Some sponges also produce “spicules” that are composed of silica.

When these organisms die, their silica skeletons fall to the bottom, dissolve, recrystallize, and might become part of a chert nodule. In some areas the sedimentation rate of these materials is high enough to produce rock layers that are thick and laterally extensive. Chert formed in this way could be considered a biological sedimentary rock.

Marble Bar Chert

Marble Bar Chert: Outcrop of the 3.4 Ga Marble Bar Chert, Pilbara Craton, Australia. The hematite-rich chert has been used as evidence of high levels of atmospheric oxygen in the early Archean. Image by NASA Astrobiological Institute.

What is Chert’s Composition?

Chert is a microcrystalline silicon dioxide (SiO2). As chert nodules or concretions grow within a sediment mass, their growth can incorporate significant amounts of the surrounding sediment as inclusions. These inclusions can impart a distinctive color to the chert.

What Color is Chert?

Chert occurs in a wide variety of colors. Continuous color gradients exist between white and black or between cream and brown. Green, yellow, orange, and red cherts are also common. The darker colors often result from inclusions of mineral matter and organic matter. Abundant iron oxides in the chert can produce a red color. The name “jasper” is frequently used for these reddish cherts. Abundant organic material can produce gray or black chert. The name “flint” is often used in reference to the darker colors of chert.

chert arrowhead

Chert Arrowhead: A chert (flint) arrowhead bound to a wooden arrow shaft with sinew. Image copyright iStockphoto / Brian Brockman.

Chert cabochons

Chert cabochons: Occasionally, specimens of chert with attractive colors or interesting patterns are cut as gemstones . These chert cabochons are examples.

Chert Used to Make Sharp Tools

Chert has very few uses today; however, it was a very important tool-making material in the past. Chert has two properties that made it especially useful: 1) it breaks with a conchoidal fracture to form very sharp edges, and, 2) it is very hard (7 on the Mohs Scale ).

The edges of broken chert are sharp and tend to retain their sharpness because chert is a very hard and very durable rock. Thousands of years ago people discovered these properties of chert and learned how to intentionally break it to produce cutting tools such as knife blades, arrowheads, scrapers, and ax heads. Tons of chert fragments have been found at locations where these objects were produced in what was one of the earliest manufacturing activities of people.

Chert is not found everywhere. It was a precious commodity that early people traded and transported long distances. As early as 8000 BC, the people of what are now England and France dug shafts up to 300 feet deep into layers of soft chalk to mine chert nodules. These are some of the oldest underground mining operations ever discovered.

chert used in a flintlock

Flintlock: A close-up of the lock of a flintlock rifle, a weapon of the 18th century used in the Revolutionary War. Note the piece of chert (flint) in the hammer. Image copyright iStockphoto / Kakupacal.

Making Fire and Sharpening Steel

Chert is a very hard material that produces a spark when it is struck against steel. The heat from this spark can be used to start fires. A “flintlock” is an early firearm in which a charge of gunpowder is ignited by a flint hammer striking a metal plate (see photo).

A variety of metamorphosed chert known as ” novaculite ” has a porous, even texture that makes it useful as a sharpening stone. The Arkansas Novaculite Formation has become world famous as a source of high-quality sharpening stones and novaculite abrasive products.

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A hard, fine-grained sedimentary rock composed of crystals of quartz (silica) that are very small
For the Spanish municipality, see Xert .
Sedimentary rock


Chert ( /ˈɜːrt/ ) is a hard, fine-grained sedimentary rock composed of crystals of quartz ( silica ) that are very small ( microcrystalline or cryptocrystalline ). [1] Quartz (silica) is the mineral form of silicon dioxide (SiO2). [2] Chert is usually of biological origin, being the petrified remains of Siliceous ooze , the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of diatoms , silicoflagellates, and radiolarians . Depending on its origin, it can contain either microfossils , small macrofossils , or both. It varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red (occasionally dark green too); 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).


  • 1 Occurrence
  • 2 Terminology
  • 3 Fossils
  • 4 Prehistoric and historic uses
    • 4.1 Tools
    • 4.2 Construction
    • 4.3 Monuments
  • 5 Varieties
  • 6 See also
  • 7 References
  • 8 External links

Occurrence[ edit ]

Flint with white weathered crust

Chert occurs in carbonate rocks as oval to irregular nodules in greensand , limestone , chalk , and dolostone formations as a replacement mineral, where it is formed as a result of some type of diagenesis . Where it occurs in chalk or marl , it is usually called flint . It also occurs in thin beds, when it is a primary deposit (such as with many jaspers and radiolarites ). Thick beds of chert occur in deep marine deposits. These thickly bedded cherts include the novaculite of the Ouachita Mountains of Arkansas , Oklahoma , and similar occurrences in Texas and South Carolina [3] in the United States . The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides .

Chert also occurs in diatomaceous deposits and is known as diatomaceous chert. Diatomaceous chert consists of beds and lenses of diatomite which were converted during diagenesis into dense, hard chert. 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. [4]

Terminology[ edit ]

Chert (dark bands) in the Devonian Corriganville-New Creek limestone, Everett, Pennsylvania

In petrology the term “chert” is used to refer generally to all rocks composed primarily of microcrystalline, cryptocrystalline and microfibrous quartz . The term does not include quartzite . Chalcedony is a microfibrous (microcrystalline with a fibrous structure) variety of quartz.

Strictly speaking, the term ” flint ” is reserved for varieties of chert which occur in chalk and marly limestone formations. [5] [6] Among non-geologists, the distinction between “flint” and “chert” is often one of quality – chert being lower quality than flint. This usage of the terminology is prevalent in North America and is likely caused by early immigrants who brought the terms from England where most true flint (that found in chalk formations) was indeed of better quality than “common chert” (from limestone formations).[ citation needed ]

Among petrologists[ example needed ], chalcedony is sometimes considered separately from chert due to its fibrous structure. Since many cherts contain both microcrystalline and microfibrous quartz, it is sometimes difficult to classify a rock as completely chalcedony, thus its general inclusion as a variety of chert.[ citation needed ]

Fossils[ edit ]

Chert layer (prominent band near top of outcrop) in the Eocene Ping Tau Formation , Hong Kong

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. [7]

For example:

  • The 3.2 Ga chert of the Fig Tree Formation in the Barbeton Mountains between Swaziland and South Africa preserved non-colonial unicellular bacteria-like fossils. [8]
  • The Gunflint Chert of western Ontario (1.9 to 2.3 Ga) preserves not only bacteria and cyanobacteria but also organisms believed to be ammonia-consuming and some that resemble green algae and fungus-like organisms. [9]
  • The Apex Chert (3.4 Ga) of the Pilbara craton , Australia preserved eleven taxa of prokaryotes . [10]
  • The Bitter Springs Formation of the Amadeus Basin , Central Australia, preserves 850 Ma cyanobacteria and algae. [11]
  • The Rhynie chert (410 Ma) of Scotland has remains of a Devonian land flora and fauna with preservation so perfect that it allows cellular studies of the fossils.

Prehistoric and historic uses[ edit ]

Tools[ edit ]

Mill Creek chert from the Parkin Site in Arkansas

In prehistoric times, chert was often used as a raw material for the construction of stone tools . 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. 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 air gun 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 an iron-bearing surface sparks result. This makes chert an excellent tool for starting fires, and both flint and common chert were used in various types of fire-starting tools, such as tinderboxes , throughout history. A primary historic use of common chert and flint was for flintlock firearms , in which the chert striking a metal plate produces a spark that ignites a small reservoir containing black powder , discharging the firearm. [2]

Construction[ edit ]

Cherts are subject to problems when used as concrete aggregates. Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. The other concern is that certain cherts undergo an alkali-silica reaction with high-alkali cements. This reaction leads to cracking and expansion of concrete and ultimately to failure of the material. [12]

In some areas, chert is ubiquitous as stream gravel and fieldstone and is currently used as construction material and road surfacing. Part of chert’s popularity in road surfacing or driveway construction is that rain tends to firm and compact chert while other fill often gets muddy when wet.

Monuments[ edit ]

Chert has been used in late nineteenth-century and early twentieth-century headstones or grave markers in Tennessee and other regions.

Varieties[ edit ]

There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics. [13] [14] Some of the more common varieties are:

  • Flint is a compact microcrystalline quartz. It is found in chalk or marly limestone formations and is formed by a replacement of calcium carbonate with silica . It is commonly found as nodules. This variety was often used in past times to make bladed tools.
  • “Common chert” is a variety of chert which forms in limestone formations by replacement of calcium carbonate with silica. This is the most abundantly found variety of chert. It is generally considered to be less attractive for producing gem stones and bladed tools than flint.
  • Jasper is a variety of chert formed as primary deposits, found in or in connection with magmatic formations which owes its red color to iron(III) inclusions. Jasper frequently also occurs in black, yellow or even green (depending on the type of iron it contains). Jasper is usually opaque to near opaque.
  • Radiolarite is a variety of chert formed as primary deposits and containing radiolarian microfossils.
  • Chalcedony is a microfibrous quartz.
  • Agate is distinctly banded chalcedony with successive layers differing in color or value.
  • Onyx is a banded agate with layers in parallel lines, often black and white.
  • Opal is a hydrated silicon dioxide. It is often of a Neogenic origin. In fact it is not a mineral (it is a mineraloid ) and it is generally not considered a variety of chert, although some varieties of opal (opal-C and opal-CT) are microcrystalline and contain much less water (sometime none). Often people without petrological training confuse opal with chert due to similar visible and physical characteristics.
  • Magadi-type chert is a variety that forms from a sodium silicate precursor in highly alkaline lakes such as Lake Magadi in Kenya.
  • Porcelanite is a term used for fine-grained siliceous rocks with a texture and a fracture resembling those of unglazed porcelain.
  • Siliceous sinter is porous, low-density, light-colored siliceous rock deposited by waters of hot springs and geysers.
  • Mozarkite has won distinction because of its unique variation of colors and its ability to take a high polish.

Other lesser used terms for chert (most of them archaic) include firestone, silex, silica stone, chat, and flintstone.

See also[ edit ]

Akcakoca chert nodules within soft limestone

  • Petrology  – The branch of geology that studies the origin, composition, distribution and structure of rocks
    • Eolith  – A chipped flint nodule
    • Nodule (geology)  – Small mass of a mineral with a contrasting composition to the enclosing sediment or rock not to be confused with Concretion
    • Obsidian  – Naturally occurring volcanic glass
    • Opal  – A hydrated amorphous form of silica
    • Whinstone  – Quarrying term for any hard dark-coloured rock
  • Archaeology  – The study of the past through material culture
    • Clovis Points , archaeological artefacts of the Clovis culture in New Mexico
    • Piatra Tomii , a prehistoric chert mine in Alba County , Romania

References[ edit ]

  1. ^ Knauth, L. Paul. “A model for the origin of chert in limestone.” Geology 7, no. 6 (1979): 274-277.
  2. ^ a b “Chert: Sedimentary Rock – Pictures, Definition, Formation” . Retrieved 2018-05-12.

  3. ^ “OpenLearn Live: 19th February 2016: A Week In South Carolina:” . OpenLearn. The Open University. Retrieved 20 February 2016.
  4. ^ *Sam Boggs, Jr., “Principles of Sedimentology and Stratigraphy”, Prentice Hall, 2006, 4th Ed., ISBN   0-13-154728-3
  5. ^ George R. Rapp, “Archaeomineralogy”, 2002. ISBN   3-540-42579-9
  6. ^ Barbara E. Luedtke, “The Identification of Sources of Chert Artifacts”, American Antiquity, Vol. 44, No.4 (Oct., 1979), 744–57.
  7. ^ The Earliest Life: Annotated listing Archived 2006-04-26 at the Wayback Machine .
  8. ^ Fig Tree Formation of South Africa
  9. ^ Gunflint chert Archived 2005-06-12 at the Wayback Machine .
  10. ^ Biogenicity of Microfossils in the Apex Chert
  11. ^ Cyanobacertial fossils of the Bitter Springs Chert, UMCP Berkeley
  12. ^ Terry R. West. “Geology Applied to Engineering,” Waveland Press, 1995 ISBN   1577666550
  13. ^ W.L. Roberts, T.J. Campbell, G.R. Rapp Jr., “Encyclopedia of Mineralogy, Second Edition”, 1990. ISBN   0-442-27681-8
  14. ^ R.S. Mitchell, “Dictionary of Rocks”, 1985. ISBN   0-442-26328-7

External links[ edit ]

Wikimedia Commons has media related to Chert .
  • Photo & note re: Fig Tree Formation
  • Microphotographs of Fig Tree fossils
  • Schopf, J.W. (1999) Cradle of Life: The Discovery of Earth’s Earliest Fossils, Princeton University Press, 336 p.  ISBN   0-691-00230-4
  • An Archaeological Guide To Chert Types Of East-Central Illinois
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      Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. It is a common rock type which occurs mostly in carbonate rocks either in nodular form or in layers (bedded chert).

      Flint at Stevns Klint
      Flint on the coast at Stevns Klint in Denmark. Gray rocks in the background are also flint boulders, but they lack fresh look because of mostly wave-agitated rounding. The boulder in the foreground is freshly broken to demonstrate typical sharp edges of chert which were eagerly exploited by our stone age ancestors.

      It is usually either dull or semivitreous. It may have many colors, depending on the nature of impurities. Most common shades are gray, white, blue, green, yellow, black, and red. White coloration is usually given by carbonate impurities; organic matter or clay gives black color; red, yellow, and brown tones are due to hematite , green variety may contain chlorite or smectite from diagenetically altered volcanic tuffs .

      It occurs usually as nodules in carbonate rocks, especially well-known are chalks with chert (flint) nodules in Western Europe. This rock is often bedded – rhythmically interlayered with chalk, shale or in some cases hematite. The latter is known as a banded iron formation (BIF) which is the principal iron ore upon which our society relies.

      Chert is in most cases a biogenic rock, it is made of siliceous tests of diatoms, radiolarians, siliceous sponge spicules, etc. Sometimes microscopic fossilized remains of these sea creatures may be preserved in these rocks. Their siliceous tests are not made of quartz initially, but after burial, compaction, and diagenesis, opaline siliceous sediments transform to quartz. Although the material it is made of ultimately came from siliceous tests of marine species, the rock itself is often not deposited in situ. It may move as a silica-rich liquid and form nodules in rocks by replacing the original (usually carbonate) material. So chert is also sometimes said to be a rock of chemogenic origin. Bedded variety seems to be often associated with turbidity currents.

      Chert nodule
      An irregularly shaped nodule from dolomite in Estonia. The width of the sample is 16 cm.

      Banded iron formations formed in Precambrian time. The formation of BIF is usually associated with the oxygenation of seawater by photosynthetic cyanobacteria, one of the earliest life forms. Exact knowledge how these old and valuable rocks formed is still lacking. It may also precipitate inorganically directly from hydrothermal solutions.

      The term “flint” is essentially synonymous with chert, but its usage is more restricted, at least in geology where “chert” is preferred. Flint is a dark variety of chert for some geologists (especially when it occurs as a nodule in chalk) and semivitreous chert has been named that way. Archeologists also talk about flint when they refer to prehistoric tools made of cherty material.

      Chert is a very hard rock and it may splinter when struck with a hammer. Warning on your hammer to wear safety goggles is very much justified if you intend to hammer it.


      Jasper is a hematite-bearing variety. Hematite is an iron oxide that is the most widespread source of red color in minerals and rocks. The sample is from the Løkken ophiolite in Norway. It is associated with SEDEX-type magnetite-hematite iron ore formed by hydrothermal activity at the spreading zones of mid-ocean ridges. Width of sample 13 cm.

      A nodule from Cyprus. The width of the sample is 10 cm.

      Chert concretion
      A rounded concretionary nodule from dolostone in Estonia. The width of the sample is 8 cm.

      Chert and chalk
      A contact between chalk and chert which exhibits characteristic conchoidal fracture. The width of the sample from Cyprus is 7 cm.

      An outcrop of bedded chert and chalk
      An outcrop of bedded chert and chalk. Governor’s Beach, Cyprus.

      Chert bed
      Chert is more resistant to weathering than chalk, so the layers tend to stand out. Governor’s Beach, Cyprus.

      Flint in chalk
      A semivitreous dark-colored nodule (flint) in chalk breccia. Governor’s Beach, Cyprus.

      Chert nodules in chalk
      Nodules in chalk. White Park Bay, Northern Ireland.

      Chert with chalk
      With chalk. Little holes in chalk are caused by tafoni weathering. White Park Bay, Northern Ireland.

      A network of chert in chalk
      A network of chert in chalk. White Park Bay, Northern Ireland.

      Prominent layers of flint nodules in chalk
      It is evident that although the nodules are diagenetic, their locations still reflect the original deposition layers of siliceous material. I have annotated most prominent layers of chert nodules in chalk with blue dots. Outcrop in a quarry in Northern Ireland.

      An elongated nodule in chalk
      An elongated nodule in chalk. Northern Ireland.

      A large chert nodule, 50 cm across
      A large chert nodule, 50 cm across, in Northern Ireland.

      Diatomaceous earth
      Diatomite (diatomaceous earth) is a soft and very light-weight sedimentary rock that is mostly composed of tiny siliceous shells of diatoms. Diatomite transforms to chert in elevated pressure and temperature conditions. The sample is from Armenia. Width of sample is 7 cm.

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