sedimentary
Chert Nodule (Flint)
Chert (Cryptocrystalline Silica - SiO2)
Hardness: 7 (Mohs scale); Color: Dark gray to black core with a tan/brown weathered cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (displays conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Dark gray to black core with a tan/brown weathered cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark gray to black core with a tan/brown weathered cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (displays conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica in sedimentary rock (often limestone or chalk) during diagenesis. Silica replaces carbonate material or fills voids. Can range from hundreds of millions to tens of millions of years old.
Uses & applications
Historically used for stone tools (arrowheads, knives) and strike-a-lights for fire making. Modern uses include road aggregate, jewelry (when colorful), and laboratory abrasive.
Geological facts
Flint is a variety of chert that occurs in chalk or marly limestone. It was the first 'high-tech' material of the Stone Age because its conchoidal fracture creates edges sharper than a steel scalpel.
Field identification & locations
Identify in the field by its distinct weathered outer crust (cortex) and glassy, shell-like fracture when broken. Found commonly in the UK (Chalk Downs), USA (Ohio, Texas), and various riverbeds. Collectors look for sharp edges and consistent color.
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