sedimentary
Chert (Nodule)
Microcrystalline or Cryptocrystalline Silica (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or bluish-gray with a waxy to dull luster; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
- Hardness
- 6
- Color
- Gray, tan, or bluish-gray with a waxy to dull luster
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or bluish-gray with a waxy to dull luster; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
Formation & geological history
Formed through the precipitation of silica in sedimentary rock layers, often replacing organic material or occupying voids in limestone. This specimen appears to be a weathered nodule, likely millions of years old, weathered out of its host carbonate rock.
Uses & applications
Historically used for making stone tools (flintknapping) due to its sharp edges; modernly used in road construction, as an abrasive, and by collectors as a lapidary material.
Geological facts
Chert is chemically identical to quartz but has a different internal structure. It has been used by humans for hundreds of thousands of years to create arrowheads, scrapers, and fire-starting tools.
Field identification & locations
Identify in the field by its hardness (it will scratch glass and steel), its smooth 'shell-like' conchoidal fracture when broken, and its spark-producing property when struck against steel. Commonly found in stream beds or eroding out of limestone cliffs.
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