sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with yellowish-orange iron staining; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic hexagonal); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, and grey with yellowish-orange iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with yellowish-orange iron staining; Luster: Waxy to dull; Crystal Structure: Cryptocrystalline (microscopic hexagonal); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from silica-rich groundwater. Most specimens found in surface deposits range from 10,000 to several hundred million years old.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road aggregate, landscape stone, and occasionally lapidary work for cabochons.
Geological facts
Chert is so durable that it often survives millions of years of erosion, becoming rounded in riverbeds long after the surrounding softer rock has dissolved. Its iron-staining (patina) occurs as groundwater rich in iron oxides permeates the microscopic pores.
Field identification & locations
Identify in the field by its hardness (it will scratch glass and steel), conchoidal (shell-like) fracture, and waxy texture when wet. Commonly found in riverbeds, glacial tills, and limestone outcrops throughout North America, especially in the Midwest.
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