sedimentary
Chert (Jasper/Flint variant)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs. Color: Variable grey, bluish-grey, and tan. Luster: Waxy to sub-vitreous. Crystal Structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Variable grey, bluish-grey, and tan
- Luster
- Waxy to sub-vitreous
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Physical properties
Hardness: 6.5-7 Mohs. Color: Variable grey, bluish-grey, and tan. Luster: Waxy to sub-vitreous. 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 radiolarians or diatoms) or chemical precipitation from silica-rich fluids in sedimentary basins. Geological age varies widely from Precambrian to Cenozoic.
Uses & applications
Historically used for making stone tools (arrowheads). Currently used as an ornamental stone, in lapidary work, and occasionally as a construction aggregate.
Geological facts
Chert with a high organic content or iron impurities can create distinct banding. It is famous for its use by prehistoric humans due to its ability to hold a sharp edge when fractured.
Field identification & locations
Identify in the field by its hardness (will scratch steel), waxy texture, and conchoidal 'shell-like' fracture patterns. Common in limestone formations and as river-worn cobbles.
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