sedimentary
Banded Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Gray, tan, and brownish bands; Luster: Waxy to sub-vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 7 Mohs scale
- Color
- Gray, tan, and brownish bands
- Luster
- Waxy to sub-vitreous
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Physical properties
Hardness: 7 Mohs scale; Color: Gray, tan, and brownish bands; Luster: Waxy to sub-vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed primarily by the biochemical or chemical precipitation of silica in marine environments, often from the accumulation of siliceous organisms like diatoms or radiolarians, or via replacement of limestone.
Uses & applications
Used historically for stone tools due to sharp edges; currently used for decorative stones, road surfacing, or as a semi-precious gemstone in lapidary work.
Geological facts
Chert is essentially the same mineral as flint, jasper, and chalcedony. The banding represents changes in the depositional environment or oxygen levels during formation.
Field identification & locations
Identifiable by its hardness (scratches glass), smooth waxy feel, and distinctive conchoidal (shell-like) fracture patterns. Frequently found in limestone or chalk beds.
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