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.

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