sedimentary
Chert (often called Flint when dark)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Dark brown to black; Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Dark brown to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark brown to black; Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids. Can be found in Precambrian to modern strata.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Today used as road surfacing, in some ceramics, and as decorative landscaping stones.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel. Dark chert is specifically referred to as flint and was essential for starting fires in the pre-industrial era.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy feel, and smooth, curved (conchoidal) breaks. Often found as nodules within limestone or chalk beds.
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