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Chert is a hard, dense, cryptocrystalline sedimentary rock composed of microcrystalline or cryptocrystalline quartz (SiO2). It is characterized by its conchoidal fracture, glassy to dull luster, and wide range of colors. It typically occurs as nodules, concretions, or beds within other sedimentary rocks, particularly limestones and dolomites. Its fine-grained nature means individual quartz crystals are not visible to the naked eye.
How to Identify
- Color
- Highly variable: white, gray, black (flint), brown, red (jasper), green, yellow, blue. Often mottled or banded.
- Luster
- Waxy, dull, or vitreous (glassy) on fresh fracture surfaces.
- Texture
- Very fine-grained (cryptocrystalline), smooth to slightly granular. Feels smooth and dense.
- Crystal Form
- Does not exhibit macroscopic crystal forms; occurs as massive, nodular, or bedded aggregates.
- Cleavage
- None. Exhibits characteristic conchoidal fracture.
- Geological Environment
- Commonly found as nodules or beds within limestone, dolomite, and chalk formations. Also occurs in deep-sea sediments (radiolarites, diatomites) and as replacement minerals in various sedimentary and sometimes volcanic rocks.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.58 - 2.64 g/cm³
- Crystal System: Trigonal (microcrystalline aggregates)
- Color: Highly variable: white, gray, black, brown, red, green, yellow, blue; often mottled or banded.
- Luster: Waxy, dull, or vitreous (glassy) on fresh fracture surfaces.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal (shell-like), producing very sharp edges.
- Cleavage: None.
- Composition: Silicon dioxide (SiO2), often with minor impurities (e.g., iron oxides, organic matter, clay minerals) that impart color.
Quick Check
- Color: Variable (white, gray, black, brown, red, green)
- Luster: Waxy to dull, sometimes vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, nodular, concretionary, bedded. Individual crystals are microscopic.
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Waxy, dull, or vitreous
Formation
Chert forms primarily through the precipitation of silica from solution, or by the diagenetic alteration and replacement of other minerals (like carbonates) by silica. It can be biogenic, forming from the accumulation and compaction of siliceous microfossils (e.g., diatoms, radiolarians, sponge spicules), or non-biogenic, forming from direct chemical precipitation from silica-rich waters, often associated with volcanic activity or hydrothermal vents. Diagenetic replacement is a common mechanism where silica-rich fluids infiltrate existing sedimentary rocks, dissolving original minerals and precipitating chert in their place. This often results in chert nodules or beds within limestone or dolomite.
Usage
Historically, chert (especially flint) was crucial for tool-making due to its conchoidal fracture, producing sharp edges. It was used for arrowheads, spear points, scrapers, and other cutting tools. In modern times, chert is used as an aggregate in concrete, road construction, and as a decorative stone. High-quality chert (novaculite) is used as a sharpening stone (whetstone). Some varieties are used as gemstones (e.g., jasper, agate).
Age Distribution
Found in sedimentary sequences from Precambrian to Cenozoic eras, with significant occurrences in Paleozoic and Mesozoic strata.
Where to Find
North America
Abundant in many Paleozoic and Mesozoic carbonate sequences, such as the Burlington Limestone (Midwest USA), Fort Payne Formation (southeastern USA), and various chert-rich units in the Appalachian and Ouachita Mountains. Novaculite is found in Arkansas and Oklahoma.
Europe
Flint is very common in the chalk formations of England (e.g., Dover Cliffs), France, and other parts of Western Europe. Chert beds are also found in various sedimentary basins.
Asia
Significant chert deposits are found in the Tethyan sedimentary sequences, particularly in regions like the Himalayas and Southeast Asia, often associated with ancient ocean basins.
Australia
Chert is present in various sedimentary successions, including Precambrian banded iron formations and younger marine deposits.
Finding Tips
Look in Carbonate Rocks
Chert nodules and beds are frequently found interbedded with or replacing limestone and dolomite. Examine outcrops of these rocks for hard, dense, often darker-colored inclusions.
Check Stream Beds and Gravel Pits
Due to its hardness and resistance to weathering, chert often accumulates in stream beds, river gravels, and glacial till. Look for smooth, rounded, dense pebbles and cobbles.
Examine Road Cuts and Construction Sites
Fresh exposures of sedimentary rock layers, such as those found in road cuts or construction excavations, can reveal chert layers or nodules.
Look for Conchoidal Fracture
A key identifying feature is the conchoidal (shell-like) fracture, which produces sharp, curved edges. This is often visible on broken pieces.
Test Hardness
Chert is hard (Mohs 7) and will scratch glass and steel. This helps distinguish it from softer rocks.
Similar Rocks
Jasper
Jasper (cryptocrystalline quartz)
Also known as: Red Chert, Green Chert
Agate
Agate (banded cryptocrystalline quartz)
Also known as: Banded Chalcedony
Chalcedony
Chalcedony (cryptocrystalline quartz)
Also known as: Cryptocrystalline Quartz
Flint
Flint (cryptocrystalline quartz)
Also known as: Black Chert
Opal
Opal (SiO2·nH2O)
Also known as: Hydrated Amorphous Silica
Scientific Classification
- Mineral Class
- Silicate (specifically, Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (referring to the constituent microcrystalline quartz)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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