Chert () is a hard, fine-grained sedimentary rock
composed of microcrystalline
) crystals of quartz
, the mineral form of silicon dioxide
Chert is characteristically of biological origin but may also occur inorganically as a chemical precipitate or a diagenetic
replacement, as in petrified wood
Chert is typically composed of 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
cherts are notable for the presence of fossil cyanobacteria
. In addition to microfossils
chert occasionally contains macrofossil
s. However, some chert is devoid of any fossils.
[Boggs 206, p.207]
Chert varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red
[W.L. Roberts, T.J. Campbell, G.R. Rapp Jr., "Encyclopedia of Mineralogy, Second Edition", 1990. ] [R.S. Mitchell, "Dictionary of Rocks", 1985. ]
(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
Chert occurs in carbonate rock
s as oval to irregular nodules
, and dolomite
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 jasper
s and radiolarite
s). Thick beds of chert occur in deep marine deposits. These thickly bedded cherts include the novaculite
of the Ouachita Mountains
, and similar occurrences in Texas
and South Carolina
in the United States
. The banded iron formation
s of Precambrian
age are composed of alternating layers of chert and iron oxide
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.
[Boggs 2006, p.206]
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
is a microfibrous (microcrystalline with a fibrous structure) variety of quartz.
[Boggs 2006, pp.206-207]
The term "flint
" is often reserved for varieties of chert which occur in chalk and marly limestone formations. 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 particularly prevalent in Great Britain where most true flint (that found in chalk formations) was indeed of better quality than "common chert" (from limestone formations).
Among petrologists e.g. Philip King
, Robert B. Neuman, Jarvis B. Hadley,
is sometimes considered separately from chert due to its fibrous structure.
nature of chert, combined with its above average ability to resist weathering
has made it an ideal rock for preservation of early life forms.
* The 3.2 Ga
chert of the Fig Tree Formation
in the Barberton Mountains between Swaziland
and South Africa
preserved non-colonial unicellular
* The Gunflint Chert
of western Ontario (1.9 to 2.3 Ga) preserves not only bacteria
but also organisms believed to be ammonia-consuming and some that resemble green algae
and fungus-like organisms.
* The Apex Chert
(3.4 Ga) of the Pilbara craton
preserved eleven taxa of prokaryotes
. These findings are now disputed.
* The Bitter Springs Formation
of the Amadeus Basin
, Central Australia, preserves 850 Ma cyanobacteria and algae.
* 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
In prehistoric times, chert was often used as a raw material for the construction of stone tool
s. Like obsidian
, as well as some rhyolite
s, and other tool stone
s 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 platform
s, bulbs of force
, and occasionally eraillure
s, 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 tinderbox
es, 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.
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.
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.
There are numerous varieties of chert, classified based on their visible, microscopic and physical characteristics.
Some of the more common varieties are:
is a compact microcrystalline quartz. It was originally the name for chert found in chalk
or marly limestone formations formed by a replacement of calcium carbonate with silica
. Commonly found as nodules, this variety was often used in past times to make bladed tools. Today, some geologists refer to any dark gray to black chert as flint.
[Bates,, R. L., and Jackson, J. (eds.) (1984). ''Dictionary of Geological Terms, 3rd Ed''. American Geological Institute. Doubleday. p. 187. OCLC 465393210]
* "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.
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.
is a variety of chert formed as primary deposits and containing radiolarian
is a microfibrous quartz.
is distinctly banded chalcedony with successive layers differing in color or value.
is a banded agate with layers in parallel lines, often black and white.
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
* Porcelanite is a term used for fine-grained siliceous rocks with a texture and a fracture resembling those of unglazed porcelain.
* Tripolitic chert (or tripoli) is a light-colored porous friable siliceous (largely chalcedonic) sedimentary rock, which results from the weathering (decalcification) of chert or siliceous limestone.
* Siliceous sinter
is porous, low-density, light-colored siliceous rock deposited by waters of hot springs and geysers.
a varicolored, easily polished Ordovician chert that takes a high polish. It is the state rock of Missouri.
Other lesser used terms for chert (most of them archaic) include firestone, silex, silica stone, chat, and flintstone.
** not to be confused with Concretion
** , archaeological artefacts of the Clovis culture
in New Mexico
** , a prehistoric chert mine in Alba County
Photo & note re: Fig Tree Formation
* Schopf, J.W. (1999) ''Cradle of Life: The Discovery of Earth's Earliest Fossils'', Princeton University Press, 336 p.
Category:Firelighting using percussion