sedimentary
Chert (Flint Pebbles)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or black with white cortex; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Fractures: Conchoidal (shell-like curves); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or black with white cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or black with white cortex; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Fractures: Conchoidal (shell-like curves); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires (striking against steel produces sparks). Today, it is used in road construction, as a gemstone (when colorful, like jasper), and for aquarium gravel.
Geological facts
Flint is a variety of chert that was the primary material for prehistoric technology due to its ability to hold a sharp edge. The white exterior coating is called a 'cortex' and is usually weathered silica or chalk residue.
Field identification & locations
Identify by its waxy appearance, extreme hardness (scratches glass), and distinctive shell-like (conchoidal) fracture pattern. Commonly found in riverbeds, glacial till, and coastal chalk cliffs.
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