sedimentary
Chert (Nodule)
Chert (primarily Microcrystalline Quartz - SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Dark brown to black. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Dark brown to black
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Dark brown to black. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or the chemical replacement of limestone/dolomite by silica. Samples like this are often found in limestone beds and can vary from Precambrian to Holocene in age.
Uses & applications
Historically used for stone tools/arrowheads due to its conchoidal fracture. Modern uses include road surfacing, jewelry (as jasper), and as a component in glass making or decorative stone.
Geological facts
Because it breaks with very sharp edges, chert and its variety flint were the primary materials for human technology for over two million years. It is extremely resistant to chemical weathering.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its curved, shell-like fracture pattern. Often found as rounded nodules in chalk or limestone outcrops or as smooth river pebbles.
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