sedimentary
Chert (and its variety Flint)
Silicon Dioxide (SiO2) - Microcrystalline or cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Variegated, often gray, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.6 – 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Variegated, often gray, tan, brown, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Variegated, often gray, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.6 – 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or sponges) or the chemical precipitation of silica in seawater/groundwater. It often forms as nodules in limestone or chalk beds across many geological ages, from the Precambrian to present.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges; used in modern industry for road surfacing, as grinding media in ball mills, and occasionally in jewelry as cabochons.
Geological facts
Flint was one of the most important minerals for early humans, used to create fire (by sparking against iron pyrites) and the first sophisticated projectile points. It is so tough that it was used as building material in flint-knapping architecture in Europe.
Field identification & locations
Identify in the field by its characteristic 'curved' shell-like (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found globally, particularly in areas with limestone or chalk deposits like the Cliffs of Dover or the Midwestern USA.
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