sedimentary
Chert (specifically Blue Chert/Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Blue-grey with white cortex; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Blue-grey with white cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Blue-grey with white cortex; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (has conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed in sedimentary environments via the accumulation of siliceous remains of marine organisms (like diatoms or radiolaria) or through chemical precipitation. It often occurs as nodules or beds within limestone or chalk of various geological ages, often Cretaceous or Jurassic.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges; currently used for lapidary work, jewelry (cabochons), road surfacing, and as a decorative stone in landscaping.
Geological facts
Chert sparked the first 'industrial' mines in human history during the Stone Age. When struck against steel, it can create sparks used for starting fires. This specific blue variety is often associated with the 'Leland Blue' slag from glass production or natural occurrences in fossil-rich limestone.
Field identification & locations
Identify in the field by its distinct conchoidal (shell-like) fracture pattern and its ability to scratch glass. It often features a white, chalky outer weathering rind (cortex). Common in the Great Lakes region (USA), UK, and various limestone-heavy mountainous regions.
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