sedimentary
Chert (and/or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Variegated tan, white, and gray; Luster: Waxy to dull; Structure: Non-crystalline mass (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Variegated tan, white, and gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Variegated tan, white, and gray; Luster: Waxy to dull; Structure: Non-crystalline mass (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or the chemical precipitation of silica in groundwater within limestone or chalk beds. It can date from the Precambrian to more recent geological eras.
Uses & applications
Historic use for stone tools and weapons (arrowheads); modern use in road construction, as an abrasive, and sometimes shaped into decorative stones or beads.
Geological facts
Chert has been used by humans for over 2 million years to create sharp-edged tools. When struck against steel, it can produce sparks, leading to its historical use in flintlock firearms.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture patterns. Frequently found in riverbeds or as nodules inside limestone outcrops worldwide.
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