sedimentary
Chert (Chalcedony)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, and creamy white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Gray, tan, and creamy white
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, and creamy white; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of siliceous remains of organisms or chemical precipitation of silica in sedimentary environments, often replacing carbonate rocks. Common throughout many geological eras from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools and arrowheads due to conchoidal fracture. Modernly used for construction aggregate, jewelry (as jasper or agate), and as a collector's specimen.
Geological facts
Chert is so hard it can create sparks when struck against steel. Flint is a well-known dark variety of chert. It was one of the first materials used by early humans for technology.
Field identification & locations
Identify by its waxy luster, smoothness, and 'shell-like' conchoidal fractures. Commonly found in limestone or dolomite as nodules. Collectors should look for sharp edges and a texture that feels more like plastic or glass than sand.
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