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Chert is a hard, dense, cryptocrystalline sedimentary rock composed of silicon dioxide (SiO2). It is characterized by its conchoidal fracture, which produces very sharp edges, and its variable color, ranging from white to black, often gray, brown, or reddish. It is typically opaque to translucent and has a dull to waxy luster. Chert is chemically resistant and durable, making it a common component of gravels and sands derived from eroded sedimentary rocks.
How to Identify
- Color
- Highly variable; commonly gray, white, black, brown, red, green, or tan. Color is often due to trace impurities (e.g., iron oxides for red/brown, organic matter for black).
- Luster
- Dull to waxy, sometimes vitreous on freshly fractured surfaces.
- Texture
- Very fine-grained (cryptocrystalline), smooth to slightly granular. Feels smooth and hard.
- Crystal Form
- Does not exhibit macroscopic crystal forms; composed of microscopic quartz crystals. Occurs as nodules, concretions, or bedded layers.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Typically found as nodules or beds within carbonate rocks (limestone, dolomite) and shales. Also occurs as bedded deposits in deep marine environments, often associated with radiolarian or diatomaceous oozes. Can be found in residual soils where carbonate host rocks have weathered away.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.58 - 2.64
- Crystal System: Trigonal (for individual quartz crystals, but cryptocrystalline aggregate)
- Color: Highly variable: gray, white, black, brown, red, green, tan
- Luster: Dull to waxy, sometimes vitreous
- Transparency: Opaque to translucent
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Variable (gray, brown, black, white, red)
- Luster: Dull to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregate; no macroscopic crystal habit. Occurs as nodules, concretions, or massive beds.
- Cleavage Type: None
- Fracture Type: Conchoidal, producing very sharp edges.
- Tenacity: Brittle
- Luster Type: Dull, waxy, or subvitreous
Formation
Chert forms primarily through the precipitation of silica from aqueous solutions, often replacing other minerals or filling voids. Biogenic chert forms from the accumulation and diagenesis of siliceous skeletal remains of organisms such as diatoms, radiolarians, and sponge spicules. Non-biogenic chert can form from the direct precipitation of silica from silica-rich waters, often associated with volcanic activity or hydrothermal vents. It commonly occurs as nodules, concretions, or beds within limestones, dolomites, and shales. Diagenetic processes, including compaction and dewatering, convert opaline silica (opal-A) to opal-CT (cristobalite-tridymite) and then to microcrystalline quartz (chert) over geological time.
Usage
Historically, chert, particularly flint, was extensively used by early humans for tool-making (e.g., arrowheads, spear points, scrapers) due to its conchoidal fracture, which produces sharp edges. It was also used for igniting fires (flint and steel). Modern uses include aggregate in construction, road surfacing, and as a decorative stone. High-purity chert (novaculite) is used as a whetstone for sharpening tools.
Age Distribution
Found in sedimentary sequences of all ages, from Precambrian to Cenozoic.
Where to Find
North America
Widespread in the Appalachian Mountains, Ozark Plateau, and throughout the midwestern and western United States in various Paleozoic and Mesozoic sedimentary formations. Notable occurrences include the Burlington Limestone (Midwest), Fort Payne Chert (Appalachians), and Arkansas Novaculite (Ouachita Mountains).
Europe
Common in chalk formations (e.g., Cretaceous Chalk of England and France, where it is known as flint), and in various Mesozoic and Cenozoic sedimentary basins.
Asia
Found in numerous sedimentary sequences, particularly in deep-sea chert formations associated with ophiolites and accretionary prisms (e.g., Japan, Himalayas).
Australia
Occurs in various sedimentary basins, including Precambrian banded iron formations and younger marine sequences.
Finding Tips
Look in Carbonate Terrains
Chert nodules are frequently found weathering out of limestone and dolomite bedrock, often accumulating in residual soils and stream beds in such areas.
Examine Road Cuts and Quarries
These exposures often reveal chert layers or nodules within sedimentary rock sequences.
Check Stream Beds and Gravel Pits
Due to its hardness and resistance to weathering, chert is a common component of river gravels and alluvial deposits.
Identify Conchoidal Fracture
A key diagnostic feature is the smooth, curved, shell-like fracture surface, often with sharp edges.
Test Hardness
Chert is hard (Mohs 7) and will scratch glass and steel.
Similar Rocks
Chalcedony
SiO2
Also known as: Agate, Onyx, Carnelian
Jasper
Microcrystalline Quartz (with iron oxides)
Also known as: Red Jasper, Green Jasper
Flint
Microcrystalline Quartz
Also known as: Black Chert
Opal
SiO2·nH2O
Also known as: Precious Opal, Common Opal
Quartzite
Metamorphosed Quartz Sandstone
Also known as: Metaquartzite
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (individual crystals are trigonal, but chert is an aggregate)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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