sedimentary
Chert and Chalcedony
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Varied (white, grey, green, reddish-brown); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None/Conchoidal fracture; Specific gravity: 2.6
- Hardness
- 6
- Color
- Varied (white, grey, green, reddish-brown)
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Varied (white, grey, green, reddish-brown); Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None/Conchoidal fracture; Specific gravity: 2.6
Formation & geological history
Formed primarily through the accumulation of silica-rich microfossils or precipitation from hydrothermal fluids in sedimentary environments and volcanic cavities. These specimens often occur as nodules within limestone or as bedded deposits.
Uses & applications
Historically used for stone tools due to its sharp edges; modern uses include decorative lapidary work, jewelry, and as an industrial abrasive.
Geological facts
Chert has been used by humans for millions of years to create arrowheads and scrapers. The reddish varieties are often called jasper, while the translucent varieties are known as chalcedony.
Field identification & locations
Identified in the field by its characteristic smooth, curved conchoidal fractures and extreme hardness (it can scratch glass). Commonly found in dry creek beds, gravel deposits, and limestone outcrops across North America and Europe.
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