sedimentary
Banded Chert (Iron-stained)
Microcrystalline Quartz (SiO2) with Iron Oxides
Hardness: 6.5-7 on Mohs scale; Color: Ochre, rusty brown, and tan banding; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 6
- Color
- Ochre, rusty brown, and tan banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Ochre, rusty brown, and tan banding; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from water, typically in marine environments. The distinct banding and coloration are caused by fluctuating levels of iron oxide (hematite and goethite) during the depositional or replacement process.
Uses & applications
Used historically for stone tools (flintknapping) due to its sharp edges; used today as landscaping stone, common road aggregate, and for making semiprecious jewelry like cabochons.
Geological facts
Chert is essentially the same mineral as jasper or flint. The iron staining in this specimen gives it a 'tiger-eye' or 'ribbon stone' appearance, though it lacks the chatoyancy of true Tiger's Eye.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, waxy surface on fresh breaks. It is ubiquitous in riverbeds and sedimentary basins worldwide.
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