sedimentary
Chert (Nodular Chert)
Microcrystalline or Cryptocrystalline Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within sedimentary host rocks like limestone or chalk. Many cherts are of biogenic origin, derived from the remains of diatoms or sponges.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; modern uses include road construction, landscaping, and occasionally as semi-precious gemstones when colorful (Jasper).
Geological facts
Chert was one of the most important prehistoric materials for survival. It is chemically identical to quartz but has a different internal structure. It can also preserve fossils in exquisite detail through a process called permineralization.
Field identification & locations
Identify in the field by its high hardness (it will scratch glass), its shell-like (conchoidal) fracture pattern, and a waxy or soapy texture on fresh surfaces. Commonly found in riverbeds and limestone outcrops.
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