sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale); Color: Brown, tan, or gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.6 - 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown, tan, or gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, tan, or gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.6 - 2.65
Formation & geological history
Formed through the accumulation of silica-rich microfossils or precipitation from silica-rich groundwater within limestone or chalk beds. Often Cretaceous to Paleozoic in age.
Uses & applications
Used historically for stone tools (flint knapping) and fire-starting; currently used in construction aggregate and as a collector's specimen.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the most important materials in human prehistory for crafting arrowheads and spear tips because it breaks with razor-sharp edges.
Field identification & locations
Identify in the field by its smooth, waxy texture and shell-like (conchoidal) fracture patterns when broken. Commonly found in riverbeds or eroding out of limestone outcrops.
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