sedimentary
Chert (and Flint)
Silicon dioxide (SiO2) - Microcrystalline quartz
Hardness: 7 (Mohs scale); Color: Tan, brown, white, or gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, white, or gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, white, or gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Often Cretaceous to Paleozoic in age.
Uses & applications
Historically used for stone tools (arrowheads, knives) and starting fires; currently used in construction aggregate and as decorative landscaping stone.
Geological facts
Chert has a conchoidal fracture, which allows it to break into edges that are sharper than a surgeon's steel scalpel. It was one of the most vital resources for prehistoric human survival.
Field identification & locations
Identify in the field by its waxy texture, extreme hardness (scratches glass), and sharp, curved fracture patterns. Commonly found as nodules in limestone outcrops.
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