sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical precipitation of silica in groundwater. Can appear in sedimentary beds or as nodules within limestone or chalk. Geological age ranges from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools and weapons (arrowheads/knives) due to sharp edges; currently used for road surfacing, as an abrasive in sandblasting, and in jewelry (tumbled stones).
Geological facts
Chert was one of the most vital materials for early humans because its conchoidal fracture pattern allows for the creation of razor-sharp edges. It is often used in 'flint and steel' fire-starting kits because it produces sparks when struck against iron.
Field identification & locations
Identify in the field by its smooth, waxy feel and characteristic curved (conchoidal) fractures that look like ripples in glass. Commonly found in riverbeds, limestone outcrops, and coastal areas. It will easily scratch glass but cannot be scratched by a steel knife.
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