What Is Chert?
Chert is a hard and compact sedimentary rock made mostly of tiny quartz crystals. It commonly occurs in carbonate rocks, either as nodules or as layered deposits known as bedded chert.
Appearance and Color
Chert is usually dull or slightly glassy (semivitreous). Its color depends on the type of impurities present. Common colors include gray, white, blue, green, yellow, black, and red.
- White: from carbonate impurities
- Black: from organic matter or clay
- Red, yellow, brown: from hematite
- Green: may contain chlorite or smectite, often from altered volcanic tuffs
The red color comes from hematite, an iron oxide. Jasper is associated with SEDEX-type iron ore formed by hydrothermal activity at mid-ocean ridges. Sample width: 13 cm.
The gray boulders in the background are also made of flint, although they appear dull due to wave-induced rounding. The freshly broken foreground nodule shows the sharp edges typical of chert, which were widely used by Stone Age people.
Occurrence
Chert is often found as nodules in carbonate rocks. A well-known example is the flint nodules in chalk formations across Western Europe. In some cases, chert appears in rhythmic layers, interbedded with rocks like chalk, shale, or even hematite.
When interlayered with hematite, the result is a banded iron formation (BIF) — a major source of iron ore that has been crucial for human industry.
Sample width: 16 cm.
Sample width: 8 cm.
Governor’s Beach, Cyprus.
Origin and Formation
Most chert is biogenic, formed from the siliceous remains of microscopic sea creatures like diatoms, radiolarians, and sponge spicules. These organisms build their tests (shells) from opaline silica, which eventually transforms into quartz through burial, compaction, and diagenesis.
In many cases, it does not form in place. Instead, silica can migrate as a liquid through rock layers and replace original materials, especially carbonates. This makes some deposits chemogenic in origin.
Bedded chert is often linked to turbidity currents, which carry sediments into deep marine environments.
Sample width: 10 cm.
Location: Governor’s Beach, Cyprus.
Governor’s Beach, Cyprus.
White Park Bay, Northern Ireland.
Small holes in the chalk are due to tafoni weathering. Location: White Park Bay, Northern Ireland.
White Park Bay, Northern Ireland.
Although the nodules formed during diagenesis, their distribution still reflects the original layers of siliceous deposition. Outcrop in a quarry, Northern Ireland.
Northern Ireland.
Northern Ireland.
Diatomite transforms into chert under elevated pressure and temperature. Sample from Armenia. Width: 7 cm.
Banded Iron Formations
Banded iron formations (BIFs) mostly formed during the Precambrian era. Their formation is commonly associated with the appearance of photosynthetic cyanobacteria, which began releasing oxygen into seawater.
The exact process remains uncertain, but in some cases, chert and iron may have precipitated directly from hydrothermal solutions.
Chert vs. Flint
The word flint is essentially a synonym for chert, but geologists often use the term “chert” more broadly. Some define flint as a dark, semivitreous variety of chert, especially when found as nodules in chalk.
In archaeology, “flint” refers to prehistoric tools made from cherty materials, regardless of the rock’s exact geological classification.
Sample from Cyprus. Width: 7 cm.
Safety Warning
Chert is a very hard rock. When struck, it can break into sharp splinters. Always wear safety goggles when hammering it — that warning label on your rock hammer exists for a good reason.
