Chert is a hard, fine-grained sedimentary rock that has been used for centuries as a tool for cutting and shaping other materials. Composed primarily of microcrystalline or cryptocrystalline quartz, chert displays an array of colors from white to black, with gray, brown, green, and red being the most common. The various hues seen in chert samples come from trace elements like iron that are present within the rock.
Chert’s classify into many varieties based on visual, microscopic, and physical traits. Some of the more prevalent forms include common chert, flint, jasper, agate, chalcedony, and novaculite. Each variety has its own distinctive features, but all share the same silicon dioxide composition and conchoidal fracturing that produces sharp edges when broken. This characteristic fracture pattern allowed early humans to craft sharp tools and weapons from chert.
Chert develops mainly as a replacement of calcium carbonate by silica in marine limestone deposits. It can also form from inorganic processes as a chemical precipitate. Chert occurs in many sedimentary settings worldwide as nodules, discontinuous bands, and interbedded layers with other rocks like shale. Its distribution depends on factors like the availability of silica, presence of calcareous sediments, and burial depth. In the United States, notable chert locations include Missouri, Pennsylvania, and Florida.
What is Chert?
Chert is a sedimentary rock made up of crystalline silica in the form of quartz and moganite. It has a hardness of 7 on the Mohs scale. Chert can be described as cryptocrystalline or microcrystalline, meaning the individual quartz crystals that compose it are too small to be distinguished with the naked eye.
The primary mineral in chert is quartz (SiO2). Quartz makes up at least 90-95% of chert in most cases. Other minerals like moganite, calcite, and iron oxide can also be present in minor amounts.
Chert frequently occurs as nodules or irregular masses embedded within limestone, dolomite, chalk, and other carbonate sedimentary rocks. It also forms in layers or bands interbedded with shales, marls, and other sedimentary deposits.
- Composition: microcrystalline quartz, minor moganite
- Color: white, gray, black, brown, green, red, others
- Streak: white
- Luster: waxy to dull
- Diaphaneity: translucent to opaque
- Cleavage: none
- Fracture: conchoidal, splintery
- Hardness: 7
- Specific Gravity: 2.6
The most distinctive physical property of chert is its conchoidal fracture. When struck or broken, it fractures unevenly to produce curved, shell-like surfaces. This creates incredibly sharp cutting edges, which is why ancient people used chert extensively for tools.
Chert’s hardness, durability, and ability to form an edge make it ideal for shaping and working other materials like wood, bone, and even softer stones.
Comparison of Chert with Other Rocks
|Chert||Microcrystalline quartz||Cryptocrystalline to microcrystalline||White, gray, black, brown, red, green||7||Conchoidal, splintery||Stone tools, gemstones|
|Flint||Microcrystalline quartz||Cryptocrystalline||Gray, brown, black, red||7||Conchoidal||Stone tools, firestarter|
|Sandstone||Quartz grains cemented together||Clastic/fragmental||White, yellow, red, brown||4-7||Irregular breakage||Building stone|
|Limestone||Calcite and aragonite||Crystalline||White, tan, gray||3-4||Irregular breakage||Building stone, cement|
|Shale||Clay minerals||Laminated, fissile||Black, brown, gray, red||3-4||Splintery||Brick, cement|
|Granite||Quartz, feldspar, mica||Phaneritic||White, pink, gray||6-7||Irregular breakage||Building stone|
|Basalt||Pyroxene, plagioclase||Aphanitic||Black, brown, gray||5-6||Columnar||Aggregate, building stone|
|Obsidian||Amorphous silica glass||Glassy||Black, brown, gray||5-6||Conchoidal||Stone tools, ornamental|
- Chert has a unique microcrystalline quartz composition
- It fractures conchoidally to produce very sharp edges
- Chert is harder and more durable than softer sedimentary rocks like limestone and shale
- It lacks the visible crystalline grains of phaneritic igneous rocks like granite
- Chert can be banded and multicolored unlike mafic volcanic rocks like basalt
- Obsidian is chemically similar but has a glassy, volcanic origin
Common Varieties of Chert
Chert has many varieties that geologists classify based on characteristics like appearance, mineralogy, and mode of formation. Here are some of the most common types:
Flint is a classic sedimentary form of microcrystalline quartz that occurs as nodules in sedimentary rocks like chalk or marine limestones. Its color is typically gray, black, brown or red due to impurities. Flint often displays banding and has a dull to waxy luster. It was one of the primary materials used to craft tools in the Stone Age.
Jasper is an opaque and impure variety of chert that displays various colors like red, yellow, green and brown. Its hue comes from the presence of iron oxides. Jasper frequently contains mineral inclusions that give it decorative patterns. Throughout history, it has been used for carvings, jewelry, and ornamental objects.
Agate belongs to the family of banded chalcedony, a microfibrous form of quartz. It exhibits distinctive color banding in concentric or parallel layers. Agate fills in cavities in volcanic rocks and is valued as a semi-precious gemstone that can be carved into beads, boxes, figurines, and more.
Onyx is a banded variety of chalcedony with straight, parallel layers of black and white. It has a smooth, wax-like finish when polished. Onyx is used for making cameos, jewelry, carvings, and ornamental objects.
Radiolarite is a siliceous, fine-grained sedimentary rock composed of the microscopic silica tests of radiolarian protozoans. It can contain trace amounts of chert, clay, or calcite. Radiolarite was used by ancient peoples for making tools and weapons.
Siliceous sinter is a porous, sedimentary rock deposited from hot springs. It consists primarily of amorphous opal-A and numerous varieties of chert like agate, chalcedony, and jasper. Notable locations include the geyser basins of Yellowstone National Park.
Mozarkite is a multicolored ornamental stone found in Missouri. It is a form of chert containing mixtures of red, pink, yellow, white, and gray.
How Does Chert Form?
There are two primary modes of chert formation:
Most chert begins forming through biochemical processes in marine environments. As siliceous organisms like radiolaria, diatoms, and sponges decay on the seafloor, their silica-rich hard parts accumulate as sediment.
Over time this silica-rich ooze lithifies into beds of chert interlayered with shale and carbonate rocks. The various macro- and microfossils in the original sediment account for chert’s common banding and color variations.
Chert also forms abiotically as a chemical precipitate from silica-saturated pore waters in sediments. This type of chert occurs as nodules, veins, and replacements of carbonate minerals like limestone, dolomite and calcite.
Factors like pH, temperature, and the concentration of silica and other elements determine what variety of microcrystalline silica is deposited. Over long periods of geological time, fluctuations in these conditions produce the distinctive banding we see in many cherts.
Many cherts are further altered through diagenetic processes after their initial formation. Fluids flowing through the sediments dissolve and reprecipitate silica, forming larger, anhedral microquartz crystals.
The minerals and textures of the original chert are often partially or completely erased. Diagenesis can introduce new colors, patterns, and textures, creating distinctive varieties of chert like agate.
Where is Chert Found?
Chert is ubiquitous in the rock record across the world. Some of the most notable occurrences are:
- Interbedded with limestone and shale in marine sediments
- Replacing carbonate minerals in limestone and dolomite
- In calcareous deep sea sediments
- In submarine volcanic deposits like ophiolites
- In hot spring and geyser deposits -Bands within banded iron formations
- In mélanges associated with subduction zones
In the United States, major chert deposits occur in Texas, Arkansas, Missouri, Iowa, Kentucky, Tennessee, and throughout the Midwest. It is also common along the outer continental shelves offshore of the Atlantic and Gulf Coasts.
Archaeologists frequently find chert tools and spearheads in America’s Midwestern plains, where ancient glacial deposits provided abundant chert nodules.
Uses of Chert
Stone Tools and Weapons
For our ancient ancestors, chert was an invaluable resource for crafting durable tools for survival. Its conchoidal fracture allowed skilled toolmakers to shape sophisticated spear points, knives, and hand axes with lethal sharpness. Archaeologists have uncovered exquisitely crafted chert tools dating back to the Stone Age.
Up until the widespread adoption of metal implements, chert remained an important raw material for tools and weapons like arrowheads, spear points, blades, and knives. Even in Colonial America, flintlock firearms used chert to ignite the gunpowder.
Chert’s hardness makes it suitable as a building material in the form of dimension stone, road aggregate, and fill. However, its variable composition and fracture means it is used less often than granite, limestone, or other rocks for construction.
Chert was a popular decorative material in medieval architecture, adorning walls, columns and monuments across Europe. Builders also added flint nodules to walls and concrete for aesthetic purposes.
Many varieties of chert like agate and jasper are valued as decorative stones that can be carved into ornaments, cabochons, beads, and other objects. Common products include figurines, eggs, spheres, vases, boxes, and bookends.
Intricately banded agates and eye-catching jaspers are often cut and polished into gemstones for jewelry. Opaque colored varieties are ideal for engraving cameos.
Chert’s abrasive qualities make it useful as a polishing compound for metals, glass, and plastics. It serves as an additive in paints, tiles, and various chemical products.
Silica derived from chert and flint is an essential raw material for manufacturing glass, ceramics, silicon metal, and silicon chemicals. Ground chert is added to clay products and Portland cement to improve hardness.
The Petrology and Mineralogy of Chert
The petrology and mineralogy of chert provide insights into its formation and unique properties. Here are some key features:
Most chert is microcrystalline, meaning the individual quartz crystals are under 10-50 microns in size and indistinguishable to the naked eye. Some varieties like flint are cryptocrystalline, with even smaller grains under 5 microns.
Quartz makes up over 90% of chert in most cases. Under a microscope, the interlocking mosaic of teeny quartz crystals give chert a dense, massive texture. Quartz provides the hardness, durability, and fracture behavior.
Minor amounts of other minerals lend chert its color and patterns. These include:
- Iron oxides – red and brown colors
- Carbonates like calcite – white to gray colors
- Clay minerals – greenish tints
- Moganite – a silica mineral unique to chert
Many forms of chert exhibit banding, swirling patterns, concentric layers, or inclusions of other minerals. These decorative textures arise from variations in composition during formation.
Chert has extremely low porosity, usually less than 1%. This results in its density, hardness, chemical resistance, and durability.
At the microscopic scale, chert consists of interlocking microcrystalline quartz grains with minor impurities. Textural variations give rise to its different varieties.
Oddities and Anomalies
Chert holds a few geological surprises:
Perfectly spherical or oddly shaped nodules of chert up to several meters wide are found embedded within limestone and dolomite. These form as silica precipitates and replaces the carbonate minerals.
While microquartz is the norm, some cherts contain much larger quartz crystals up to 2 cm long. Megaquartz chert retains the trademark hardness and fracture of regular chert.
Eoliths are oddly shaped chert nodules that resemble human or animal artifacts. Some believe Neolithic people shaped these into tools, while others argue they form naturally. The debate continues!
Rare microbial communities like bacteria, algae, and lichens can colonize the surface of chert, excreting acids that etch the rock and produce stunning natural art. These “living rocks” illustrate the important role microbes play in weathering.
The Many Uses of Chert Through History
For thousands of years, humans recognized the potential of chert for fashioning stone tools. Chert proved invaluable for survival up until the widespread adoption of metals.
Paleolithic Stone Tools
Some of the earliest known stone tools were crude flakes and hand axes carved from chert by Homo habilis around 2.6 million years ago. Hominins relied on chert’s sharp cutting edges while hunting, butchering, and processing food.
By the Neolithic period, toolmakers had mastered the art of flintknapping, producing finely worked spear points and arrowheads. Microblades carved from flint and other cherts allowed construction of composite tools like harpoons and fish hooks.
Chert produces sparks when struck against iron pyrite or steel. Scraping chert against flint produces an even bigger shower of sparks to start a fire. For centuries, chert was an ever-ready firestarter for use while camping, hunting, or mountain climbing.
Many Native American tribes used chert to craft ceremonial objects like breastplates, smoking pipes, gorgets, and effigy figures. Large chert deposits in Ohio provided raw material for Hopewell culture handicrafts.
Prior to percussion caps, firearms used chert to ignite the gunpowder. Flintlock muskets had a “frizzen” covered in a sheet of flint that would produce sparks to fire the charge when struck by the cock.
The chert-rich sediments of Northern France provided abundant material for decorative flint nodules set into walls, columns, and floors in medieval structures. Flint mosaic art adorns several English churches.
Frequently Asked Questions about Chert
What is chert?
Chert is a hard, fine-grained sedimentary rock composed primarily of microcrystalline quartz. It has a waxy to dull luster and breaks with a conchoidal fracture.
How does chert form?
Most chert forms biochemically as siliceous ooze on the ocean floor made up of the silica skeletons of diatoms and radiolarians. It can also form inorganically by direct precipitation of silica from saturated waters.
What is the difference between chert and flint?
Flint is a variety of chert that forms in chalk or limestone. There is debate over whether flint and chert should be considered distinct rocks.
What gives chert its colors?
Trace elements like iron oxide lend chert various colors from white and gray to green, red, brown, and black. Organic carbon produces black, while iron produces red and green.
Does chert have any crystals visible to the naked eye?
No, chert is microcrystalline or cryptocrystalline, meaning the individual quartz crystals are microscopic. Rarely, some cherts contain larger crystals.
Is chert fragile?
No, chert is very durable and hard with no cleavage planes. Its hardness of 7 makes it resistant to abrasion.
Can chert be polished?
Yes, many varieties of chert like agate and jasper take an excellent polish and have been used as gemstones and decorative objects.
Where are some notable locations to find chert?
Chert occurs worldwide, but notable deposits are found across the midwestern US, in Northern Europe, and offshore along continental shelves.
What is petrified wood?
Petrified wood is formed when silica replaces the organic material in wood, essentially turning it to stone. The result is wood with the composition of chert.
How was chert important in human history?
For millennia, chert’s conchoidal fracture allowed early humans to fashion sharp tools and weapons for survival until the adoption of metals.
Chert’s unique properties have made it invaluable to countless generations of toolmakers, jewelers, and builders. Even today, this remarkable sedimentary rock has many important uses. Chert serves as a geological time capsule that unlocks secrets of our planet’s past environments and ecosystems. From its microscopic grains to curiosities like giant nodules, chert is a fascinating rock with an intriguing story to tell.