sedimentary
Chert (often called Flint or Jasper)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, cream, and grey; Luster: Waxy to dull; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6–2.7
- Hardness
- 6
- Color
- Brown, tan, cream, and grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Brown, tan, cream, and grey; Luster: Waxy to dull; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6–2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from groundwater within sedimentary rock layers, often limestone or chalk. Can range from Precambrian to recent ages.
Uses & applications
Historically used for stone tools and weapons due to sharp conchoidal edges; currently used for landscaping, road ballast, lapidary art, and fire-striking kits.
Geological facts
Chert is so durable it survives for millions of years as river gravel long after the limestone it formed in has eroded away. 'Flint' is technically a dark grey to black variety of chert often found in chalk.
Field identification & locations
Identify in the field by its waxy texture, smooth conchoidal (shell-like) fractures when broken, and extreme hardness—it will easily scratch glass and even some steel.
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