sedimentary
Chert Nodule (with Geode cavity)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, and grey with iron staining; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (Conchoidal fracture).
- Hardness
- 6
- Color
- Brown, tan, and grey with iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, and grey with iron staining; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (Conchoidal fracture).
Formation & geological history
Formed as rounded masses of silica that precipitated in sedimentary rock layers, such as limestone or chalk, often through the replacement of organic material or filling of voids. Typically Cretaceous to Carboniferous in age.
Uses & applications
Used historically for tool-making due to its sharp edges; today used for lapidary work, decorative garden stones, and as geological teaching specimens.
Geological facts
Chert nodules like this one were the primary source of flint for prehistoric humans. The hollow centers occasionally contain small quartz crystals, forming a mini-geode.
Field identification & locations
Identify by its waxy texture, extreme hardness (it will scratch glass), and rounded, pebble-like exterior. Commonly found in riverbeds and eroded limestone cliffs.
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