sedimentary
Chert (variant Flint/Novaculite)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: gray, tan, yellowish-brown; Luster: waxy to dull; Crystal Structure: Microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.6 – 2.7
- Hardness
- 7 (Mohs scale)
- Color
- gray, tan, yellowish-brown
- Luster
- waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: gray, tan, yellowish-brown; Luster: waxy to dull; Crystal Structure: Microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.6 – 2.7
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of siliceous biological remains (like diatoms or radiolarians) in marine or lake environments. Often found as nodules or layers within limestone and chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include abrasives, road construction material, and semi-precious lapidary work.
Geological facts
Chert was one of the most important raw materials for early humans. Its ability to produce sparks when struck against steel led to its use in 'flintlock' firearms. 'Novaculite' is a high-grade chert from Arkansas famous for making whetstones.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy feel, and characteristic shell-like (conchoidal) fracture edges. Commonly found in limestone outcrops or as smooth river pebbles. Collectors look for unique banding or 'picture' patterns.
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