sedimentary
Banded Chert (Ironstone type)
Silicon Dioxide (SiO2) with Iron Oxide inclusions
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and dark grey banding; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Brown, tan, and dark grey banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and dark grey banding; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments; the banding indicates fluctuating mineral deposition over time. Typically found in Precambrian to Paleozoic formations.
Uses & applications
Used historically for stone tools due to sharp edges; currently used for lapidary work, tumbling, and as decorative gravel.
Geological facts
Chert is so durable that it often survives as stream pebbles long after the surrounding rock has eroded. Some varieties with high iron content are related to Banded Iron Formations (BIFs), which are some of the oldest rocks on Earth.
Field identification & locations
Identify by its hardness (it will scratch glass), smooth waxy feel when wet, and visible sedimentary layering. Commonly found in riverbeds and limestone regions.
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