sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, and brown banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, and brown banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, and brown banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed in sedimentary environments via the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or through chemical precipitation in the form of nodules or bedded deposits within limestone or chalk.
Uses & applications
Historically used for making stone tools (arrowheads, knives) and strike-a-lights; currently used as road aggregate, architectural stone, and occasionally in jewelry as lapidary material.
Geological facts
Because it breaks with a conchoidal fracture (smooth, curved surfaces), it can be knapped to create edges sharper than a steel scalpel. It was one of the first minerals extensively traded by early humans.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (scratches glass), and distinctive shell-like (conchoidal) fracture patterns when broken. Common in limestone outcrops.
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