sedimentary
Chert (specifically Yellow/Brown Chert or Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Yellowish-brown, tan, or grey; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (hexagonal system at microscopic level); Cleavage: None (has characteristic conchoidal fracture); Specific Gravity: 2.6 – 2.7.
- Hardness
- 7 on Mohs scale
- Color
- Yellowish-brown, tan, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Yellowish-brown, tan, or grey; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (hexagonal system at microscopic level); Cleavage: None (has characteristic 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 via chemical precipitation in sedimentary environments. Common in marine limestone and chalk deposits from various geological ages, notably the Cretaceous and Paleozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road surfacing, decorative garden stones, and as a raw material in ceramics. It is also a popular beginner rock for lapidary tumbling.
Geological facts
Chert has a unique property where it produces sparks when struck against steel, which made it vital for prehistoric survival and early firearms (flintlocks). The color is often derived from trace amounts of iron oxides like goethite or hematite.
Field identification & locations
Identified in the field by its hardness (it will scratch glass), smooth waxy feel, and 'shell-like' conchoidal fractures. Commonly found in riverbeds, gravel pits, and coastal cliffs. Collectors should look for unique banding or fossil inclusions.
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