sedimentary
Chert (Tan/Yellow variety)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on the Mohs scale; Color: Tan, beige, or pale yellow; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
- Hardness
- 6
- Color
- Tan, beige, or pale yellow
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on the Mohs scale; Color: Tan, beige, or pale yellow; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.65
Formation & geological history
Formed by the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater. It can be found in limestone or chalk beds as nodules or layers, ranging from Precambrian to Cenozoic ages.
Uses & applications
Historically used for stone tools and arrowheads due to its conchoidal fracture. Modernly used in construction aggregate, road surfacing, and as a polishing media in rock tumblers.
Geological facts
Chert is so hard it can strike sparks against steel, making it a primary component in 'flint and steel' fire starters. In its darker gray or black form, it is commonly known as flint.
Field identification & locations
Identify in the field by its characteristic smooth, curved (conchoidal) breaks that resemble broken glass and its ability to scratch glass. It is commonly found in sedimentary basins, riverbeds, and as nodules within limestone outcrops.
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