Chert: Formation, Types, Properties, Uses

Chert is a hard, fine-grained sedimentary rock composed primarily of microcrystalline quartz, or silica. It is typically composed of the skeletons of diatoms, radiolarians, and other siliceous organisms. Chert is often found in marine sedimentary rocks but can also occur in lacustrine and terrestrial settings.

Chert is typically found in nodules, concretions, or layered with chalk and limestone deposits, and it exhibits a wide range of colors, including white, gray, black, brown, and red.

Chert has been extensively used by prehistoric cultures for making stone tools due to its conchoidal fracture, which allows it to break with very sharp edges. Today, it's still utilized in some industries, including construction and landscaping, and it's often used as gravel or crushed stone.

Chert specimens exhibiting different colors and textures, resulting from variations in mineral composition and depositional environments.

Chert Formation

Chert forms in a variety of ways, including:

Biogenic silicification: This is the most common way that chert forms. Chert can form from the accumulation of silica skeletons of tiny organisms such as diatoms, radiolarians, and silicoflagellates. These organisms live in the ocean and their silica skeletons sink to the bottom of the seafloor. Over time, the skeletons are compacted and cemented together to form chert nodules or beds.

Chemical precipitation: Chert can also form inorganically as a chemical precipitate. This can happen when seawater becomes supersaturated with silica, or when silica-rich solutions are forced upwards into the crust. Inorganically formed chert is typically found in association with hydrothermal vents or volcanic deposits.

Diagenetic replacement: Chert can also form when other minerals, such as calcite, dolomite, and clay minerals, are replaced by silica during the process of diagenesis. Diagenesis is the chemical and physical changes that rocks undergo after they are deposited. This process, known as silicification, occurs when silica-rich fluids or solutions replace the original minerals.

Chert from the Tertiary of Utah, USA.
Photo: James St. John

Types and Varieties of Chert

Chert can be classified into several types based on its texture, composition, and origin. Some of the most common types of chert include:

Types of Chert Based on Texture

Bedded chert: This type of chert is formed when layers of chert are interbedded with layers of other rocks, such as shale or limestone. It is often found in marine sedimentary rocks.

Nodular chert: This type of chert is found in nodules, or rounded lumps, within other rocks. It is often found in chalk and other sedimentary rocks.

Radiolarian cherts in the Mesozoic of California, USA.
Photo: James St. John

Partly Lithified Siliceous Sediments: Siliceous sediments partially consolidated, exhibiting some degree of rock-like consistency but not fully lithified, often found in soils or as surface coatings.

Unlithified Siliceous Sediments: Siliceous sediments that have not undergone any consolidation, remaining as loose deposits, often found in marine and freshwater environments.

Compositional Subtypes of Chert

Radiolarite: Radiolarite is a type of chert that is composed primarily of the siliceous remains of radiolarians, which are microscopic marine organisms. It is often found in deep-sea sediments.

Diatomite: Diatomite is a type of chert that is composed primarily of the siliceous remains of diatoms, which are microscopic algae that flourish in marine and freshwater environments. It is known for its light, porous texture.

Spiculite: Spiculite is a type of chert that is composed primarily of the siliceous remains of sponge spicules, which are small, needle-like structures that form the skeleton of sponges.

Jasper: Jasper is a type of chert that is hard, opaque, and often colorful. It is often reddish, yellow, or brown in color. It is formed through the precipitation of silica around iron oxide impurities.

Varieties of Chert Based on Specific Characteristics

Porcellanite: Porcellanite is a type of chert that is very hard and has a porcelain-like appearance. It is often used to make ornamental objects.

Silcrete: Silcrete is a type of chert that is formed in soils. It is often used for building materials.

Flint 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. Today, some geologists refer to any dark gray to black chert as flint. The dark color is from inclusions of organic matter.

Mozarkite is a form of chert that is found in the Ozark Mountains of Missouri. Mozarkite is named after the Missouri Ozarks, where it is found. The rock was first described in the early 1800s by geologist Henry Rowe Schoolcraft. Mozarkite is typically found in nodules or veins within other rocks, such as limestone or dolomite. Mozarkite is a popular decorative stone. It is often used in countertops, fireplaces, and other interior and exterior applications.

Chert Nodules, Emerald Parkway Roadcut, Dublin, Ohio
Photo: James St. John

Chert Properties

Color: Chert can be found in a variety of colors, including white, gray, brown, red, and yellow. The color is typically determined by the impurities present in the rock, such as iron oxide or manganese oxide.
Grain Size: Chert is typically a microcrystalline rock. This means its grains are very small, usually less than 30 micrometers and often too small to be seen with the naked eye.
Streak: White
Translucency: Translucent to opaque
Fracture: Chert typically has a conchoidal fracture, which means that it breaks into smooth, curved surfaces like a shell.
Hardness: Chert is a very hard rock, ranking between 6.5 and 7 on the Mohs scale of mineral hardness. It is slightly softer than quartz but harder than most other sedimentary rocks.
Weathering Resistance: Chert is highly resistant to weathering due to its hardness and composition.
Texture: Chert has a fine-grained texture, with a smooth, glassy surface. It can be translucent or opaque, and it often has a distinctive conchoidal fracture, meaning it breaks with a smooth, curved surface.
Density: 2.65 g/cm³

Depositional Environment

Chert forms in various environments, but most commonly in marine settings. Here are some common depositional settings for chert:

Seafloor sediments: Microscopic organisms like radiolarians or diatoms with silica shells can accumulate on the seafloor. Over time, the silica dissolves and reprecipitates, forming chert layers within the seafloor sediments.

Volcanic activity: Hydrothermal solutions associated with volcanic activity can be rich in silica. These solutions can percolate through rocks and replace minerals with silica, forming chert deposits.

Replacement of limestone: Chert can form by replacing limestone deposits. As silica-rich fluids move through limestone, they can dissolve the calcium carbonate and replace it with silica, forming chert nodules or beds within the limestone.

Chert Uses

Chert has been used by humans for centuries for a variety of purposes, including:

Chert tools and Arrowhead

Stone Tools: Chert was widely used by prehistoric cultures for making tools and weapons due to its hardness and ability to fracture with sharp edges. These tools include arrowheads, spear points, knives, scrapers, and axes.

Flintknapping: Flintknapping is the art of shaping chert or similar materials into tools and decorative objects through controlled fracturing. It is still practiced today by hobbyists, artisans, and experimental archaeologists.

Fire Starting: Chert's ability to produce sparks when struck against steel has made it useful for starting fires. In survival situations, chert can be used in conjunction with a piece of steel to create sparks for ignition.

Sharpening: Chert was also used to sharpen other tools, such as arrowheads and knives.

Jewelry and Ornamental Use: Chert with attractive colors and patterns can be polished and used in jewelry and ornamental objects.

Industrial Applications: Chert is used in various industrial processes, such as in the production of ceramics, as an abrasive in grinding and polishing operations, and as a component in the manufacture of glass and silicon dioxide.

Chert is a fascinating and versatile sedimentary rock with a rich geological history and diverse applications in human culture.