Chert is a hard and compact sedimentary rock, consisting dominantly of very small quartz crystals. Chert is a common rock type, it occurs mostly in carbonate rocks in nodular form or in layers (bedded chert).
Chert is usually either dull or semivitreous. Chert 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 chert may contain chlorite or smectite from diagenetically altered volcanic tuffs.
Chert occurs usually as nodules in carbonate rocks, especially well-known are chalks with chert (flint) nodules in Western Europe. Chert is often bedded. It consists of rhythmically interlayered beds of chalk and chert, shale and chert, or in some cases hematite and chert. 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 chert. These siliceous tests are not made of quartz initially but after burial, compaction, and diagenesis, opaline siliceous sediments transform to quartz. Although the material chert is made of ultimately came from siliceous tests of marine species, chert 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 chert seems to be often associated with turbidity currents.
An irregularly shaped chert 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. The chert in banded iron formations is probably not of biogenic origin. Exact knowledge how these old and valuable rocks formed is still lacking. Chert may also precipitate inorganically directly from hydrothermal solutions.
The term “flint” is essentially synonymous with chert but it’s 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 chert but I recommend to avoid it altogether. Chert is most likely stronger than you.
Pictures of chert
A chert nodule from Cyprus. The width of the sample is 10 cm.
A rounded concretionary chert nodule from dolostone in Estonia. The width of the sample is 8 cm.
A contact between chalk and chert. Chert exhibits characteristic conchoidal fracture. The width of the sample from Cyprus is 7 cm.
A limestone from Germany with bluish chert nodules. The width of the sample is 17 cm.
A breccia of limestone and carbonate chert from Germany. The width of the sample is 14 cm.
Red chert is known as jasper. The width of the sample from Australia is 12 cm.
An outcrop of bedded chert and chalk. Governor’s Beach, Cyprus.
Chert is more resistant to weathering than chalk, so chert layers tend to stand out. Governor’s Beach, Cyprus.
A semivitreous dark-colored chert nodule (flint) in chalk breccia. Governor’s Beach, Cyprus.
Chert nodules in chalk. White Park Bay, Northern Ireland.
Chert in chalk. Little holes in chalk are caused by tafoni weathering. White Park Bay, Northern Ireland.
A hard chert nodule standing out in chalk. White Park Bay, Northern Ireland.
A network of chert in chalk. White Park Bay, Northern Ireland.
It is evident that although chert 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 chert nodule in chalk. Northern Ireland.
A large chert nodule, 50 cm across, in Northern Ireland.
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.