sedimentary
Chert (Nodule)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Brown, tan, and white; Luster: Dull to waxy; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Brown, tan, and white
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale; Color: Brown, tan, and white; Luster: Dull to waxy; Crystal structure: Trigonal (microscopic); 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 chemical precipitation in sedimentary environments. Often found as nodules in limestone or chalk beds. Ages range from Precambrian to Cenozoic.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative landscaping stone, in road construction as aggregate, and occasionally for lapidary work.
Geological facts
Chert is so hard it can scratch steel and glass. In historical contexts, it was a primary material for 'flint' strike-a-lights to create fire. The variety seen here with a white outer layer is often called 'chert with a cortex'.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture when broken and its extreme hardness. Often found in riverbeds or eroded from limestone cliffs. Collectors look for unique banding or fossil inclusions.
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