sedimentary
Chert (often called Flint)
Chert (Silicon Dioxide, SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, or black; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or black; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains such as radiolaria or diatoms on the ocean floor, or through chemical precipitation from groundwater within limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) and as fire-starters. Industrially used as an abrasive and in gemstone tumbling.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges thinner and sharper than a surgical steel scalpel. It has been used by humans for over 2 million years.
Field identification & locations
Identify by its waxy texture, extreme hardness (it will scratch glass), and the shell-like (conchoidal) curves where it is broken. Commonly found in limestone outcrops or as river pebbles.
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