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Chert is a microcrystalline or cryptocrystalline rock composed mainly of silica in the form of chalcedony. It forms very sharp edged flakes when struck by another rock. Darker forms are called flint.
Sedimentary: Forms as nodules or layers within other sedimentary rocks due to the accumulation of the hard skeletons of microscopic organisms (therefore technically an 'organic sedimentary' rock).
Source location: Canterbury.
Sizes: Choose from dropdown menu either Hand specimen (150-300g), or Small specimen (less than 150g).
Note: Images are examples of specimens only. Sizes and shapes will vary.
More detail
Structure
Chert is a microcrystalline or cryptocrystalline sedimentary rock composed mainly of silica (silicon dioxide). It often forms as nodules or layers within other sedimentary rocks and is derived mainly from the microscopic remains of diatoms, radiolarians and sponges.
Type
Organic sedimentary.
Physical properties of chert
- Colour: Can exhibit various colours, including white, grey, brown, black, or red, depending on impurities and mineral content. Flint is a type of chert.
- Lustre: Often dull or waxy.
- Hardness: A hard rock, ranking around 7 on the Mohs scale of mineral hardness.
- Density: Ranges from 2.5 to 2.8 grams per cubic centimetre.
- Fracture: Exhibits a conchoidal fracture, breaking with smooth, curved surfaces resembling the interior of a seashell.
General properties of chert
- Composition: Primarily composed of silica, often in the form of microcrystalline quartz.
- Microcrystalline Structure: Has a very fine-grained or cryptocrystalline structure, with crystals too small to be seen with the naked eye.
- Nodules and Layers: Typically occurs as nodules or layers within other sedimentary rocks, such as limestone or chalk.
- Chemical Inertness: Chemically inert and resistant to acids and alkalis, making it highly stable.
- Weathering Resistance: Highly resistant to weathering and erosion, maintaining its structural integrity over time.
Uses of chert in New Zealand and worldwide
- Early Māori: Chert (whaiapu) was used to make flake and cutting tools for scraping and cutting, and for making fish hooks.
- Flintknapping: Used for flintknapping, the ancient art of shaping it into arrowheads, knives, and other stone tools due to its hardness and ability to hold a sharp edge.
- Construction: Sometimes used as a construction material, particularly for decorative stone in walls, pathways, and landscaping.
- Geological Studies: Studied by geologists to understand sedimentary processes, ancient environments, and the history of Earth's crust.
- Water Filtration: Can be crushed and used as a filtration medium for water treatment, removing impurities and particulates.
- Jewellery and Decorative Objects: Chert with attractive colours and patterns may be used for jewellery, beads, cabochons, and decorative objects.