sedimentary
Chert (and/or Flint)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale. Color: Tan, brown, gray, or black. Luster: Waxy to dull. Crystal Structure: Trigonal (microscopic). Cleavage: None; exhibits a prominent conchoidal (shell-like) fracture. Specific Gravity: 2.6 – 2.7.
- Hardness
- 7 on the Mohs scale
- Color
- Tan, brown, gray, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on the Mohs scale. Color: Tan, brown, gray, or black. Luster: Waxy to dull. Crystal Structure: Trigonal (microscopic). Cleavage: None; exhibits a prominent conchoidal (shell-like) fracture. Specific Gravity: 2.6 – 2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical precipitation from silica-rich fluids within limestone or chalk beds. Prevalent throughout most geological eras, from the Precambrian to the present.
Uses & applications
Historically used for making sharp stone tools (arrowheads, knives) and for starting fires (flint and steel). Currently used in road surfacing and as a component in some ceramics and glass.
Geological facts
Chert was one of the most important minerals for early humans due to its ability to fracture into extremely sharp edges. It is often found as nodules or layers within sedimentary rock formations. Famous varieties include 'Novaculite' and 'Flint'.
Field identification & locations
Identify in the field by its hardness (it scratches glass), its waxy feel, and its sharp, curved fracture edges. Frequently found in riverbeds, limestone quarries, or coastal cliffs. Common worldwide.
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