sedimentary
Chert (Common Opal/Jasper variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale; Color: Tan, beige, and grey with possible pinkish banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Tan, beige, and grey with possible pinkish banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale; Color: Tan, beige, and grey with possible pinkish banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6–2.7.
Formation & geological history
Formed by the precipitation of silica from groundwater or within sedimentary rock layers, often replacing limestone or accumulation of radiolarians/diatoms. Common in Paleozoic to Cenozoic strata.
Uses & applications
Historically used for stone tools (knapping for arrowheads); currently used as semiprecious stones in jewelry, lapidary cabochons, and as a decorative landscaping rock.
Geological facts
Chert is so hard it can produce sparks when struck against steel. It has been used by humans for millions of years to create cutting tools due to its sharp conchoidal edges.
Field identification & locations
Identify by its 'flint-like' waxy luster, smooth conchoidal fracture (curved surfaces like broken glass), and inability to be scratched by a steel knife. Commonly found in riverbeds or eroding out of limestone cliffs.
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